A thorn is a sharp, pointed spike that grows on the stems of certain plants, especially roses and berry bushes. Thorns protect plants from animals that might eat them. If you've ever reached for a beautiful rose without being careful, you know how effectively those tiny spikes defend their flower.

Gardeners wear thick gloves when pruning rose bushes to avoid getting scratched. Wild blackberry bushes have especially fierce thorns that can tear clothing and skin, making it challenging to harvest their sweet berries. The thorns on an acacia tree in Africa can grow several inches long, strong enough to puncture a tire.

The word thorn appears in many expressions because it captures the idea of something small that causes persistent irritation. When someone describes another person as a thorn in their side, they mean that person creates ongoing annoyance or difficulty, like a classmate who constantly distracts others. If something is a thorny problem, it's complicated and difficult to solve, full of potential ways to get hurt if you're not careful.

Interestingly, cacti don't actually have thorns. Those spikes are modified leaves called spines, though most people casually call them thorns anyway.