Playing with water is what we did when we were kids on the beach. Among other fascinating discoveries from our childhood is how to use water carried in our buckets to mix it with the (dry) sand and be able to build our wonderful sand castles. And as children we soon understood that it was not necessary to add much, because then we would obtain unusable mud. We also learned that if we filled the bucket to the top and reached in, the water immediately seeped out and spilled around us. We had just discovered Archimedes’ principle empirically. The same one we used in our first experiment.
The water displaced by our head is exactly its volume. But how do we know the mass? Let’s remember our school days: mass divided by volume is the density of a body. Then, if we know the density of our head, we can know what mass it has. In a first approximation, our body has a density similar to that of water (that is why we can play “dead” in the pool and we don’t go to the bottom like stones). So the rule is easy: 1 liter is 1 kilo.
The second experiment is a classic that we usually explain wrongly. The fact that the water rises inside the glass is not due to the fact that the oxygen disappears due to the combustion of the flame. If this were the case, the water would enter continuously as it disappears (and turns into carbon dioxide, so oxygen does not end up disappearing). The underlying mechanism is that of temperature. When the flame is lit, the temperature of the gas rises and expands. When it is turned off, the temperature drops and the enclosed gas contracts and the water enters the glass.
