1What water does when you push it
Make room, and the water pushes back
You only need two ideas. Watch each one:
You have to shove water aside
Anything you dunk has to push water out of the way to fit. A wide shape shoves aside a LOT of water. A small tight shape shoves aside hardly any.
Floating is a tug-of-war
Weight pulls down. The shoved-aside water pushes up. If a shape can shove aside water that weighs as much as the object, it settles until the up-push matches its weight — and floats.
2Same foil, two shapes
The tight ball vs the wide boat
Take one piece of foil. You can crush it small or spread it wide — same metal, same weight, two very different shapes:
The tight ball
Packed into a tiny lump, it shoves aside only a thimble of water — way less than the metal weighs.
The wide boat
Opened out wide, it can shove aside a big boat-sized chunk of water — as much as the metal weighs. So it settles just enough to float.
3Your turn — load the boat
Pile on cargo and watch the up-push keep up
Here's a boat already floating. Drop crates in and watch it ride lower — sinking down just enough to shove aside extra water, so the up-push grows to match every new crate. Watch the two numbers race each other.
4Now change only the water
Same sealed boat. Could different water hold it up?
This small boat has a sealed deck, so water cannot enter. It weighs 5 g and sinks in fresh water. Its shape and weight stay fixed. We replace the surrounding water with a concentrated salt solution that weighs 20% more per spoonful.
Predict before changing the water
The same sealed 5 g boat sinks in fresh water. Could it float in the denser salt solution without changing its shape or weight?