Closure: Normally, when you exit a function, all its variables “disappear”. This is because nothing needs them anymore. But what if you declare a function inside a function? Then the inner function could still be called later, and read the variables of the outer function. In practice, this is very useful! But for this to work, the outer function’s variables need to “stick around” somewhere. So in this case, JavaScript takes care of “keeping the variables alive” instead of “forgetting” them as it would usually do. This is called a “closure”. While closures are often considered a misunderstood JavaScript aspect, you probably use them many times a day without realizing it!