This is not really one for Tech Log, but Gravity is the answer, and ambient atmospheric pressure required to keep the liquid in a liquid state.
In the weightless conditions of space, but in a pressurised capsule, siphoning will not work (the fluid would also become "free floating" bubbles).
Try siphoning a liquid from one container to another in normal atmospheric pressure. Works fine when the first container is held above the other lower vessel. Now raise the lower vessel above the first, and the fluid will transfer in the opposite direction.
Really, this one is for the "Questions" forum.
Regards,
Old Smokey