Why can't my friend sink when I hold my legs in the water?

The density of human body is slightly less than that of water, so people will definitely float in the water.

Try touching the floor at the bottom of the pool with your hands.

That is, in shallow water, take a deep breath, dive into the water first, lift your legs and feet off the water, and immediately stroke the bottom of the pool with breaststroke.

Try a few more times and you will succeed.