Taste is sort of like a set of keys and locks.
When you taste chocolate, that’s a chocolate chemical, or “key”, that comes in and turns the chocolate “lock” on your tongue. That sends a message to your brain telling you that you tasted chocolate.
What this means is you can only taste “keys” that fit your tongue’s “locks”.
You have locks for a bunch of things on your tongue. We have sugar locks to detect sugar, salt locks to detect salt, etc.
We *don’t* however, have any locks for water. So, when water comes into your mouth, there’s no way for the water to turn any locks and send a message to your brain. No message to your brain, no taste.
Water is tasteless because for us it’s more important to notice if water tastes anything else but tasteless. A bad tate would indicate there’s something wrong with it, therefore should be avoided.
Food has taste because there’s so many varieties of it, so there can’t really be a neutral, but water is the only liquid we need, therefore it’s the default taste.
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