1The secret inside the water
Water is hiding air — and ice doesn't want it
You only need two ideas. Watch each one:
Water is full of invisible air
Tap water has air dissolved right inside it. Those tiny bubbles that creep up the side of a glass of water? That's the hidden air slowly sneaking out.
Ice shoves the air out
Growing ice tends to reject dissolved gases and many impurities. As water turns to ice, the freezing edge pushes the air out in front of it. The air has to go somewhere.
2Two ways water can freeze
The trap vs the sweep
It all comes down to how the ice grows. There are two ways, and they treat the air completely differently:
The trap
Ice forms on every side and slams shut in the middle — sealing the air inside as tiny bubbles.
The sweep
One growing edge pushes much of the dissolved air into the remaining liquid. The air has not vanished: that last liquid still has to go somewhere.
3Your turn — find the hidden air
Watch dissolved gas leave water
This is water with more dissolved air than it can keep under its new conditions. Move the slider to see bubbles form and leave. Ordinary stirring can also add air; it does not reliably remove all the dissolved gas.
4Now freeze it — two ways
What if we freeze the last water too?
Both cups freeze slowly from the top down. In cup A, stop before the bottom freezes and remove the last liquid. In cup B, keep freezing everything. Predict what happens to the bottom of B.
Guess before you freeze
The top-down ice is pushing air into the remaining water. If cup B freezes all the way to the bottom, what happens there?