Video narration
What makes a rule count as a function? Imagine a classroom number machine. You put a number in, follow the machine's rule, and an answer comes out. Say the rule is "add two." Put in one, and out comes three. Put in two, and out comes four. Put in three, and out comes five. That's the machine doing its job consistently. Each input has one clear output, so the machine is behaving like a function. Now suppose you put in the same number, but the machine gives you two different answers. That rule fails the test. The key isn't whether answers repeat; it's whether one input can get two different answers. Different inputs can share an output, and the machine can still be a function. So what makes a rule count? A function is a number machine where every input gets exactly one output.