An apple is made up of apple things, water, and loads of air bubbles(real low density, in other words, light). Coins on the other hand, are made out of metal(real high density, in other words, heavy). read more
water has a density of 1. For something to float, the density of the object would have to be under 1, and to sink, it must be over 1. If it is exactly 1, then the object will remain suspended in the water. A penny has a density of over 1, so it sinks in water. read more