Q: What do chemists call a benzene ring with iron atoms replacing the carbon atoms? A: A ferrous wheel.

Explanation: A benzene ring is a hexagon-shaped ring made out of hydrogen and carbon atoms — so it basically resembles a wheel. "Ferrous" is used an adjective to describe something with iron in it, so a wheel of iron is a Ferrous wheel, which sounds similar to Ferris wheel, the carnival ride.