A killjoy is someone who ruins other people's fun by complaining, criticizing, or refusing to join in. Picture your class planning an exciting field trip to the science museum, and one student keeps saying “That sounds boring” or “I don't want to go” or pointing out everything that could go wrong. That's killjoy behavior.

A killjoy at a birthday party might grumble about the music being too loud. A killjoy at recess might refuse to play and tell everyone else that their game is stupid. A killjoy sibling might complain through the entire family movie night.

Sometimes people act like killjoys without meaning to. They might be having a bad day, or they genuinely don't understand why others are excited. But persistent killjoys drain the energy from a room. They focus on problems instead of possibilities, on what's wrong instead of what could be fun.

Being cautious or raising real safety concerns doesn't make you a killjoy. There's a difference between saying “Maybe we shouldn't climb that tree, it looks dangerous” and saying “Climbing trees is stupid and boring anyway.” One shows good judgment; the other just spoils the mood.