Okay kiddo, have you ever opened a bottle of soda or juice and it squirted everywhere? That's because there were a lot of bubbles inside it trying to get out. Sometimes we don't want all the bubbles to escape at once, so we use something called a "fizz-keeper".
A fizz-keeper is a little device that you put on top of your bottle after you open it. It has a special pump on it that you press down a few times to remove some of the air from the bottle. This makes it so the bubbles in the soda or juice don't get as big and intense, so they stay inside the bottle instead of making a mess.
Think of it like a lid that you put on your bottle, but it also helps keep the bubbles inside. And the best part is that you can use it over and over again on different bottles. So if you don't finish your drink all at once, you can use the fizz-keeper to save it for later without having it go flat or explode all over the place. Pretty neat, right?