Why should you always knock on the fridge door before opening it?
The salad could be dressing!
Listen up, dummy! This joke is funny because it uses a pun. "Dressing" has two meanings: it's what you put on a salad, but it also means putting on clothes. So the joke is saying the salad might be getting dressed, which is unexpected and silly. I know you are not used to unexpected outcomes in life since you still think you will be rich one day.