Chlorine
Chlorine is a strong-smelling chemical added to tap water and pools to kill germs and make water safe to drink.
Think of it this way
A few drops of bleach in a bucket of water act like chlorine in a tap. The bleach spreads through the water and kills germs floating inside it. Each drop reaches far more water than it touches directly. The bucket stays clear, but the germs are gone.
Explaining chlorine by grade level
Tap water has a tiny bit of a germ killer mixed in. You can sometimes smell it at a pool. A carbon filter in a water pitcher can take that smell out. Over time the filter wears out and stops working as well.