A ganglion cyst is a fluid-filled sac that forms as a herniation from a joint capsule or tendon sheath. The sac is attached to the joint or tendon sheath by a "stalk" that allows fluid to move into the pouch from the joint or sheath. The stalk functions as a valve and often limits fluid drainage out of the cyst, allowing the cyst to increase - but not decrease - in size. In some cases the stalk functions as a two-way valve, allowing fluid to travel in both directions. This can enable the cyst to increase and decrease in size based on activities.