..."one of humanity’s greatest inventions: the hot bath. Perfect for idle contemplation and the occasional Archimedes-style ‘Eureka!’ moment. Showers might save water but they don’t give you time to think. So there you are, lying in the bath with your favourite popular-science book, the scent of lavender wafting in the steam, Vivaldi in the background and a mug of peppermint tea on the corner of the tub. Bliss. You even manage to keep the pages dry when your mind wanders for a second and you wake with your mouth dipping below the surface. Hmm. Lavender soap doesn’t taste as good as it smells.
Still, everything else is perfect. If you cool off, that’s easily fixed. Just add more water from the hot tap and carry on doing nothing. The hot water plummets to the bottom because of gravity, then rises to the top as it’s less dense – it has fewer molecules in a given volume than the rest of your now too-cool-for-comfort bathwater. As it gains height, the hot water pulls colder, denser water from the other end of the bath to the place it’s just left. The result is a convection current that mixes everything up and distributes the heat without you having to lift a finger (though a quick swoosh with your hand works wonders even more quickly).
Convection – the second way of transferring heat – occurs in all liquids and gases because their atoms or molecules can move about freely. Conduction, in contrast, works best in solids, where the atoms or molecules are confined near set positions and tend to be closer together, although it can take place in liquids and gases too. So you can thank two types of physics for warming you up in the bath: convection moves the hot water to you and conduction gets the heat into your body, just as it warms a she-male garter snake in a mating ball. All’s well again.
Eventually your fingertips become pale and wrinkled and you decide to get out. Disaster! As you heave yourself out of the tub, water dribbling down your body, you realise your towel is in the wicker laundry basket in your bedroom. The one your aunt gave you. Curses. It was toasty warm in the bath but now you’re freezing as you scurry across the corridor, leaving sodden footprints on the carpet."