Answer:
What has a bell but isn't a church. Is full of air but is not a balloon?
Salty water everywhere but not sea in sight.
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.
What goes up but never comes down?
Describes universities like Harvard and can be poisonous.
Who makes it, has no need of it. Who buys it, has no use for it. Who uses it, can't see it. What is it?