What are Taste and Smell?

Taste and smell are more or less the same process as each other: you have sensors in your mouth and nose which can recognise different classes of chemical. There are five different kinds of taste: sweet, sour, bitter, salt and the recently discovered "umami" (monosodium glutomate - the Japanese have actually known about this fifth kind of taste for a very long time, but it took a long time for the rest of the world to catch on). All the tastes of different foods are made out of combinations of these five fundamental flavours, but that's only true if you ignore the role of smell. Whenever you eat anything, chemicals are released from the food and they travel up into the nose through the inside route from the back of the mouth: smell is actually an important part of taste, and food would taste pretty bland without it. While the tongue tastes flavours on direct contact with stuff that you put in your mouth, smell works on individual molecules which drift about in the air: these molecules have to collide with sensors in the nose which feel their shape and see if they fit any known patterns. If they don't, we don't smell them at all, so we can only smell the molecules which we are designed to be able to detect. Some things smell nasty to us because they are dangerous: rotting food is unsafe to eat, but luckily it releases distinctive molecules which we recognise as unpleasant and so we avoid eating or even touching anything with those kinds of smell. These molecules which drift through the air are themselves too small to be dirty or germ-ridden, so these nasty smells are themselves harmless: it's contact with the rotting substance that is dangerous. There are however some gases which smell bad and are indeed dangerous, so it is best to play safe and get well away from anything that smells bad, but animal smells will do you no actual harm, even if they are disgusting.
Smoke smells pretty distinctive, and there are always bigger particles in smoke which are dangerous, so you should always try to avoid breathing it in. Unfortunately, people have spent at least a hundred thousand years living with fire and sleeping in smoke-filled huts: we are not designed to be alarmed easily by smoke. A person who is asleep will typically remain asleep if their house goes on fire during the night and they will not be woken even by thick smoke. Poisonous fumes in the smoke will then kill them in their sleep long before the fire reaches them. This is why it is essential that your house should have at least one working smoke detector in it, and preferably two. If you wake up early enough, it should be easy to get out alive. You should consider the possibility that you may need to be able to escape through any window in the house, whether that's upstairs or downstairs, so plan your escape now rather than waiting until there is a fire. Never sleep in a room with barred windows unless there are two different ways out of it. If the ground is a long way down from a window that you might need to escape through, make sure you have a plan as to how you're going to get down safely. I would never sleep high up in a building without a rope that I could abseil down, and I would never go high up inside a skyscraper unless I had a base-jumping parachute with me: these things should really be provided as standard equipment in all tall buildings, and the people who live and work in them should be trained in how to use them. It really disappoints me whenever I see disasters in the news with fires killing people in tall buildings when just a little bit of preparation could have saved them all.

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