Stamp glue activates with water because the adhesive is dried onto the paper in a thin layer.

The longer version

The adhesive on a stamp is applied wet and then dried into a hard film. In that state it is inert and does not stick to the sheet behind it. Adding water redissolves the surface of the film, turning it tacky again so it can flow into the paper fibre and grip as it dries a second time under gentle pressure.

Later issues added self-adhesive backings that need no water, but early gum had its own quirks: it could crack in dry climates, curl the stamp, or pick up flavour from the materials used. That is why old unused stamps sometimes show a brownish crust on the back, and why collectors handle them with tongs rather than fingers.

Where this entry sits

Odd Little Things looks at the ordinary objects people touch every day and the small mechanisms and histories hiding inside them. Buttons, zips, pencils, mirrors, stairs, stamps, packaging, and road signs all get their turn. The full list sits on the Odd Little Things page, where every entry is listed in order.