Round with a Ring
In order to fill a bathtub full of water you must have a way to keep the water from going down the drain. A bathtub stopper can do this. There are a few types of bathtub stoppers that you can use. The first is the bathtub stopper that is round with a small ring at the top. You can see an example of it in the picture above. The stopper is placed over the bathtub drain hole. It covers the hole and keeps the water from going down the drain allowing you to fill up your bathtub. The ring gives the individual something to pull on when they are ready to release the water and exit the tub.
More modern bathtubs come with a bathtub stopper that screws in to place. The idea is to keep the bathtub stopper in the drain at all times. Then you just screw it in to make it tight enough to keep the water in the top, and unscrew it to let the water out. There is a rubber ring around the edge of the bathtub stopper so that when you screw the stopper down, the rubber ring forms a seal to keep the water out. This type of bathtub stopper should be effective, but in many cases it doesn't do all that well.
Some bathtubs have a built-in system. There is a metal switch underneath the bathtub faucet. If you push the metal switch up, it locks the drain up so that the water cannot come out. When you push the switch down, the drain is unlocked and the water is free to exit down the drain. This type of bathtub stopper also has a metal piece above the drain that is used to catch hair and other small objects that might be able to fall down into the drain and get stuck in the pipes.