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Everything's perfect, you've been enjoying your bath for more than 20 minutes now. Nice haze around the room, bath smells good, you made yourself a nice moustache with the foam, nothing could make you happier. But, here it comes, you feared it from the beginning. It's happening, as usual. It never fails. And I'm like you: I hate it when the bath gets cold!

This is all about heat moving all around you. Water is hot, the air and the tub aren't. As nature always tends to balance, this whole system will eventually have the same temperature. Water is the heat source. The tub and the air are the heat sinks. But those two sinks "eat" heat in different ways. The tub is in contact with water. The same way you heat up your feet after a winter walk with your hands, water is heating up the tub. This is called conduction, heat transfer by contact.

The air just over the water surface is being heated. As you know, hot air is lighter than cold air. Thanks to Archimedes, we know that this hot air will go up. And the nearby cold air replaces it. This new air is being heated in turn... and so on. This is called convection, heat transfer by displacement of fluid. When all the air will share the same temperature, this movement will stop.