Spiny means covered with sharp, pointed parts called spines. A cactus is spiny, with needles that prick your finger if you touch them carelessly. A porcupine is spiny, protected by thousands of quills. Sea urchins, certain caterpillars, and many desert plants are all spiny creatures and organisms that use their sharp points for defense.

Spines work like natural armor. A spiny lizard's scales make it painful for predators to swallow. A spiny fish like a lionfish warns other animals to stay away. These sharp points protect vulnerable animals and plants from being eaten or trampled.

The word can also describe someone's personality or mood. When your normally cheerful friend acts spiny after a bad day, they're being prickly and irritable, snapping at innocent questions. A spiny problem is one that's difficult to handle, with complications sticking out in every direction, like a porcupine's quills.