Fluid encapsulated in rock: possible / common?

Hey all, while reading up on meteorites (unrelated to question) I remembered a specimen from my childhood which should be buried in a box at my parents house right now (thus no photos):

It looked pretty much like such geode but it was not cut open to display crystals. Instead, one could see some kind of fluid slosh around inside when holding it up to a lightsource.

Is something like this common? What kind of fluid would / could be encapsulated inside a rock?

Sorry for not having photos at hand - I'll probably have my parents send me this specimen and several meteorites soon.

edit: thanks to all for clearing this up!

TL;DR: Pretty common, search for enhydros. :P