Answer:
In we go, out we go. All around and in a row. Always, always steady flow. When we'll stop, you'll never known. In we go, out we go.
Who makes it, has no need of it. Who buys it, has no use for it. Who uses it can neither see nor feel it. What is it?
So cold, damp and dark this place. To stay you would refrain, yet those who occupy this place do never complain.
What can be swallowed, but can swallow you?
The sun bakes them, the hand breaks them, the foot treads on them, and the mouth tastes them.
The sun bakes them, The hand breaks them, The foot treads on them, And the mouth tastes them. What are they?