Why is glue sticky?
For grown-ups: the science
Glue spreads into tiny spaces, then dries into a solid layer that holds surfaces together.
Many common water-based glues contain polymer particles or dissolved polymers. While wet, the adhesive wets surfaces and flows into microscopic roughness. As water evaporates or is absorbed, the polymer forms a continuous solid film that adheres through intermolecular attractions and mechanical interlocking; different glues cure by different chemical or physical processes.
What about...?
Ask your child's own question
Transcript
A drop of glue sits between two pieces of paper. Why does it make them stay together?
Wet glue spreads into tiny bumps and spaces on both surfaces. It touches lots of little places at once.
As glue dries, much of its water goes into the air. The glue changes into a solid layer.
That solid layer grips both surfaces and holds them together. Pulling the pieces apart takes more force.
So glue is sticky because it spreads, dries, and makes a gripping layer between things.