A live wire is an electrical wire that's currently carrying electricity and could give you a dangerous shock if you touch it. Electricians work carefully around live wires, using special tools and protective equipment. When you see warning signs about live wires, they're telling you to stay back because the electricity flowing through those wires is active and dangerous right now.

The phrase also describes a person who's full of energy and excitement, always moving and doing things. If your teacher calls you a live wire, she means you're energetic, enthusiastic, and hard to keep still. A live wire student might be the first to volunteer, quick with ideas, and constantly engaged in what's happening around them.

The connection between these meanings makes sense: just as electricity flows intensely through a live wire, energy and enthusiasm seem to flow through an energetic person. Someone might say “She's a real live wire” about a friend who's always organizing games, suggesting activities, and keeping everyone entertained.