What’s in the glue that seals envelopes?

Assuming you are talking about the glue that needs moisture to seal the envelope. This type of glue is made from gum arabic, which is a product of the hardened sap from two different kinds of acacia tree, which can be found in west Africa and northwestern India. It is edible for humans and has E number E414. As well as being useful for sealing envelopes it is also a staple ingredient in certain sweets such as M&Ms, marshmallows and gumdrops. Non-edible applications also include the binding of watercolour paint and an important ingredient of shoe polish. Pretty useful stuff.
Author: Rik Sargent, Science Museum