Children love sweet things to eat, and when mom or dad bakes some fresh cookies, it's a real treat. Of course, lots of cookies means some of them must be put away for later days. Many kitchens have a cookie jar for that purpose. Parents may warn their kids, "Don't take any cookies from the cookie jar without permission!"

 

However, children may secretly try to do that. If they don't get caught, they are happy, but if their parents catch them with a hand in the cookie jar, they get into trouble. Today, we can use this idiom in any situation where someone is caught doing something they should not do.