This short riddle asks you to think of something that can be opened only once and can never truly be closed again afterward.
The answer is an egg.
When you crack an egg open, the shell separates and the inside is exposed. You can place the broken pieces back together, but you cannot restore the egg to its original sealed state. In that sense, once the egg has been opened, it cannot really be closed again.
The wording is clever because “open” usually makes us think of doors, boxes, books, or containers that can easily be shut again. An egg behaves differently. Its shell is a one-time seal, so opening it permanently changes it. That simple twist is what makes this familiar riddle work.
