Two people are alone in a desert, and they have one full bottle of water between them. The water bottle is clear and plastic. The water bottle has been deformed by the heat and hence it is hard to gauge how much liquid it actually holds. They agree to each drink half of it each.
The first person takes a drink, then hands the bottle to the second person. However, the second person thinks the first person drank more than their fair share.
How can they check? They haven’t got any other containers, so they can’t pour any of the water out.
Person two notes where the water level is on the bottle. They can do this by making a mark, or simply keeping their finger there. They then turns the bottle upside down (with a lid on). If there the water level is below the place that was noted earlier, then person one drank more than their fair share.