The buoyancy of water
Find a metal can like a coffee can or a cigarette tin which has a tightly fitting cover. With the cover on, push the can into a pail of water, cover end down, and rapidly let go of it. Repeat this having the can in dissimilar positions. What do you observe? Can you observe the up thrust on the can? Put a little water in the can and repeat the experiment. Keep adding water a little at a time and repeating unless the can no longer floats.