Bladder

Bladders ought to be a fairly familiar concept, a stretchy membrane for containing fluids/gases. Bladders are relevant to space technology because they are used inside fuel tanks for at least 2 good reasons. Firstly, they confine the fuel into a volume roughly equal to the volume of fuel remaining, which prevents it from sloshing around and causing disturbances to the attitude control or GNC systems. Also, in a microgravity environment, it means you can keep the fuel in a partially filled tank close to the exit valve so you can use it. Easy really. Check out this page from Astrium about their bladder tanks.