Q: Our third grade class would like to know why pine cones close when they
are soaked in water.

A: Are you prepared to explain the mysteries of osmotic pressure?

Basically, the cones close up because the cells at the base of each scale absorb water …and the increased water pressure in the cells cause the scale to fold forward.

When the cells dry out, the scales fold backward and open the cone up.

The reason this benefits the tree is that the dry, mature cone effectively releases its seed so the pine can procreate.

Now, if you really want to have some fun with this, get them to soak someone’s hand in a plastic tub of water and give you their theories why their skin wrinkles up.

For a hint, see:
Fingers Wrinkle