Asked by Rick Francis
There are many reasons for this, and it all depends on the materials in the fabric. Wool is made of animal hair that is covered with scales. With heat and water these scales start to stick out and as the jumper spins around, the fibres lock together. You can imagine a long strand getting more entangled, and therefore shorter. other materials shrink as well. During the process of making fabric and clothes there is nothing shrinking them, but they’re always stretching. Heat allows fibres to move more freely and as they rearrange themselves, your clothes will change.
José Monteiro, Science Museum Explainer