To happen means to take place or occur, usually without being planned. When something happens, it comes into existence in the real world. An accident might happen on the highway. A funny coincidence might happen when you run into your teacher at the grocery store. Things that happen are events, moments, or situations that become real rather than staying imaginary or possible.

Unlike words like create or arrange, which suggest someone deliberately made something occur, happen often implies that events unfold on their own or through circumstances beyond anyone's control. A thunderstorm happens. A friendship happens when two people naturally start spending time together. You might plan a birthday party, but you can't plan the unexpected things that will happen during it.

When you ask “What happened?” you're asking someone to explain events that already took place. When you say something happened to you, you mean you experienced it, like “I happened to find a dollar on the sidewalk.” If someone happens upon a discovery, they find it by chance rather than through searching.