How many glasses of water do we need to drink everyday why?

Answer 1

Exactly how much depends on your size, your activity, your surrounding temperature, and what else you're eating and drinking.

A common recommendation is that you should drink 8 glasses of water each day, which is a bit of a simplification.

It'd be more correct, but not as catchy to say that you need to get the equivalent fluid of 8 glasses of water each day. And you get some fluids with what you eat. A bowl of milk and cereal will count for instance.

Why is because we're always losing fluid.

We lose some when we breathe, we lose some through sweating. There's going to the toilet too. And as water make up a large proportion of our body weight, the body works best when the amount of water in it is about right.