What can be opened but cannot be closed?

Why this works

This riddle plays on the idea of opening and closing in a literal sense. An egg can be opened to access the contents inside, like the yolk and egg white, but once it's opened, you can't "close" it back up in the same way. The fun twist lies in the fact that while we typically think of things that can be opened as also being closable, in this case, the egg's nature defies that expectation!