We need to take in water every day to replenish the water our bodies use, but where did we ever get the idea that we need eight glasses of water a day? No one knows, exactly. It was probably made up by the same guy who said you can’t go swimming for 30 minutes after you eat. You should try to take in several glasses of water a day—and if you wait until you’re thirsty for water, you’re probably already dehydrated. But eight is not a magic number. If you eat a lot of fruits and veggies, you are probably getting a lot of water from solid foods—and most of us never count that toward our total water intake.