In one sentence, describe what the following article is about:

Water-tight is airtight, and you need to prevent air from escaping your hands to make them whistle. You lips will be puckered out, but somewhat close together. Imagine you are "booing" someone on stage.

Summary:
Cup your left hand like you are about to drink water from it. Purse your lips as if you were making an "ooo" sound.