How can it float in the air ?
The balloon is filled with helium gas which is lighter than air. It is similar to when we blow air under water. The bubbles filled with air under the water would go up because air is much lighter than the surrounding water.
My question is about the phrases "under water" and "under the water". Is it okay to leave out the "the" in the first usage? And what are the rules to include or exclude "the" in this situation?