I am sure you must have done this yourself at some stage. A quick and fun experiment to set up. You just need a glass, water and coins, we used ¥1.
Fill the glass right to the top with water, so that it is almost over-flowing. The challenge is to see how many coins you get in the glass without the water spilling over. Ebi-kun tried this several times, I think his best score was 14. Fun bit of kitchen science, perfect to kill some time!