What is penny joke?

As an AI language model, I can answer that a penny joke usually refers to a type of joke that is very short and often perceived as cheap or "costing only a penny." These jokes are often puns, one-liners, or wordplay that can be shared quickly and easily. Here's an example: "Why don't scientists trust atoms? Because they make up everything." Penny jokes are often used as icebreakers or to lighten the mood in social settings.