Microwaves are a kind of electromagnetic wave and as such can create electric currents in metals. Many of the microwaves are actually reflected by the metal and can reflect back onto the magnetron – the part of the oven that produces the microwaves, which can overheat and become damaged. When microwaves hit metal surfaces, free electrons (negatively charged particles) in the metal start to move around and the movement of this charge is how an electric current arises. Some of these electrons will move too much and will actually jump from the metal to the air which becomes temporarily ionised (charged). This can result in a phenomenon called ‘arcing’ where an electric spark similar to a flash of lightning is produced. Thin metal such as rims of mugs, produce more resistance to an electric current than thick metal, and so can become very hot. If sufficiently thin the metal can become so hot that it actually melts!

Cells are the building blocks of all living organisms. Individual cells are classified as living things, and there are millions of organisms that are unicellular across the planet.

As they are living units, cells consequently need energy, and therefore respire to survive. Parts of the cell, called organelles, work like organs of a body. Energy for the cell to process can be provided by the cell, such as through photosynthesis in plants, or absorbed into the cell through cell membranes and then processed within it by the mitochondrion. Single cells operate like this, and there are billions of unicellular organisms that survive independently or within multicellular organisms. These single cell organisms are generally prokaryotic cells, which are much smaller and have fewer organelles, most importantly lacking a nucleus. Multicellular organisms are primarily made up of eukaryotic cells which are more complex and can therefore specialise so the organism can become more complex. They do this by grouping together to form tissues, which then group to form organs within the organism.

Cells reproduce to replace old, damaged cells in an organism, to allow growth or growth of a new individual. In unicellular organisms, cell reproduction is obviously the only way a population will grow. Prokaryotes favour binary fission, where all genetic information is doubled and then the cell divides into two new, identical cells. Eukaryote cells use either mitosis, which results in two identical organisms or cells, or meiosis, which results in each new cell having half the number of chromosomes of the original cell.

Human hair grows on average 1.25 centimetres (0.5 inches) per month, which is equivalent to about 15 centimetres (six inches) per year. There are several variables that can affect hair’s growth rate such as age, health and genetics. Each hair grows in three stages, the first being the anagen phase where most growth occurs. The longer your hair remains in this stage dictates how long and quickly it develops; this can last between two and eight years and is followed by the catagen (transitional) and telogen (resting) phases. Hair growth rates vary across different areas of the head, with that on the crown growing the fastest.

Answered by Laura Mead, Science Museum.

At the centre of every atom is a nucleus containing protons and neutrons. Together, protons and neutrons are known as nucleons. Around this core of the atom, a certain number of electrons orbit in shells. The nucleus and electrons are referred to as subatomic particles. The electrons orbit around the centre of the atom due to the charges present; protons have a positive charge, neutrons are neutral and electrons have a negative charge. It is the electromagnetic force that keeps the electrons in orbit due to these charges, one of the four fundamental forces of nature. It acts between charged objects – such as inside a battery – by the interaction of photons, which are the basic units of light.

An atom is about one tenth of a nanometre in diameter. 43 million iron atoms lined up side by side would produce a line only one millimetre in length. However, most of an atom is empty space. The nucleus of the atom accounts for only a 10,000th of the overall size of the atom, despite containing almost all of the atom’s mass. Protons and neutrons have about 2,000 times more mass than an electron, which results in the electrons orbit the nucleus at a large distance as they in turn have an extremely low mass.

An atom represents the smallest part of an element that can exist by itself. Each element’s atoms have a different structure. The number of protons inside a specific element is unique. For example, carbon has six protons whereas gold has 79. However, some elements have more than one form. The other forms – known as isotopes – will have the same number of protons but a different number of neutrons. For example, hydrogen has three forms which all have one proton; tritium has two neutrons, deuterium has one neutron and hydrogen itself has none.

As different atoms have different numbers of protons and neutrons, they also have different masses, which determine the properties of an element. The larger the mass of an atom the smaller its size, as the electrons orbit more closely to the nucleus due to a stronger electromagnetic force. For example an atom of sulphur, which has 16 protons and 16 neutrons, has the same mass as 32 hydrogen atoms, which each have one proton and no neutrons.

(Image credit: Science Photo Library)

Humans use the energy in food to live; this energy is measured in calories. If people consume lots of calories from food and don’t burn them up, they get converted to fat and stored in the body and so people gain weight. This was very important when humans were not always guaranteed access to food daily. They could use the stored fat in their bodies for energy until they could find food again. If people burn more calories than they consume through food, by doing lots of exercise etc, then their bodies use the fat stores instead and, as a result, they lose weight.

Answered by Kate Mulcahy, Science Museum.

We stop growing because we are genetically programmed to. Our genes, from our parents, determine how we develop but environmental factors also affect how tall we grow. We know our DNA controls the growing process, but scientists are still unsure how our genes do this. We stop growing when we reach the age at which our bodies can reproduce. At this point, the purpose for growth is complete so our genetic control centre tells us to stop.

Answered by Louise Thomas, Science Museum.

Einstein’s Theory of Special Relativity, first postulated in 1905, says that the laws of physics and the speed of light are the same for all observers, regardless of their own speed or motion. To have a better understanding, imagine two people travelling at different speeds observing the same beam of light. According to Special Relativity, both will record the same speed for the beam, regardless of their own speed and direction.

This contradicts more practical examples on Earth. If a car moves at 40mph away from an observer, and another travels at 50mph from the same point in the same direction, relative to each other the second car will be moving at 10mph. However, a light beam moving in the same direction as the cars would appear to have no change in speed relative to them and would remain at its universally agreed value c, about 299,792,458 metres per second. Of course, this is theoretical and in practice not measurable, but nevertheless it’s the basis of Einstein’s Theory of Special Relativity.

The biology of the eye is extremely complex, especially when you consider the human eye only has the rough diameter of 2.54 cm and weighs approximately 7.5 grams. It is made up of around 15 distinct parts, all with different roles to play in receiving light into the eye and transmitting the electrical impulses, which ultimately relay image information to our brains so that we can perceive the world we live in.

The eye is often compared to a basic camera, and indeed the very first camera was designed with the concept of the eye in mind. We can reduce the complex process that occurs to process light into vision within the eye to a relatively basic sequence of events. First, light passes through the cornea, which refracts the light so that it enters the eye in the right direction, and aqueous humour, into the main body of the eye through the pupil. The iris contracts to control pupil size and this limits the amount of light that is let through into the eye so that light-sensitive parts of the eye are not damaged.

The pupil can vary in size between 2 mm and 8 mm, increasing to allow up to 30 times more light in than the minimum. The light is then passed through the lens, which further refracts the light, which then travels through the vitreous humour to the back of the eye and is reflected onto the retina, the centre point of which is the macula.

The retina is where the rods and cones are situated, rods being responsible for vision when low levels of light are present and cones being responsible for colour vision and specific detail. Rods are far more numerous as more cells are needed to react in low levels of light and are situated around the focal point of cones. This focal gathering of cones is collectively called the fovea, which is situated within the macula. All the light information that has been received by the eye is then converted into electrical impulses by a chemical in the retina called rhodopsin, also known as purple visual, and the impulses are then transmitted through the optic nerve to the brain where they are perceived as ‘vision’. The eye moves to allow a range of vision of approximately 180 degrees and to do this it has four primary muscles which control the movement of the eyeball. These allow the eye to move up and down and across, while restricting movement so that the eye does not rotate back into the socket.

Despite its small size, the skunk can expertly defend itself against predators, such as bears, that are much larger than itself. There are few things that will deter a predator more than an offensive odour, and the skunk, a small mammal native to North America, is probably most notorious for this ability.

Beneath the skunk’s tail are two internal walnut-sized glands that produce a foul-smelling oily spray that can be ejected up to three metres (ten feet). The pungent substance is a thiol, a strong-smelling organic sulphur compound, contact with which can result in a burning or stinging sensation in its victims. While this is not particularly damaging, it is the horrendous stench that is most offputting – it sends out a message to would-be predators that this creature doesn’t taste good, so stay away.

Skunks will only release their spray if they feel really threatened as the glands only hold enough of the pungent concoction for five or six strikes and it can take up to ten days to replenish. The animal gives plenty of warning before letting off a stink bomb, including stamping its feet and thrusting its tail high in the air in preparation. When ready to spray, the skunk lifts its tail and extends a tiny protrusion from each gland from which the noxious scent is emitted. Muscles around the glands enable the spray to be projected quickly and with high precision.