In its most basic form, sharks and minnows is a game of water tag. The game starts with one shark and several minnows. The shark lurks in one end (or the middle) of the pool. The minnows stand at the other end. When the shark yells "Minnows in!" or a similar phrase, the minnows jump or dive into the pool and try to swim to the other side without getting tagged by the shark. Every minnow the shark tags joins the shark's team and helps to tag other minnows. The last minnow becomes the first shark in the next round.
There are rule variations about when a minnow can be tagged. In one version, a minnow cannot be tagged if she is fully under the water. In that case, the shark can either wait for the minnow to surface for a breath, or the shark can try to pull her to the surface to make the tag.
In another version of the game, stealth is critical. The game starts with the shark facing away from the pool. The shark can only turn around when she hears a minnow in the water. Minnows who enter the water silently and swim without splashing are rewarded with a head start toward the other side.
Safe zones are also popular. If a minnow is touching the agreed-upon safe zone (like the pool drain or diving board), the shark can't tag it.