Help the chief engineer complete the warning label on the space station's new espresso machine.

The zero-gravity espresso machine will overheat and spray hot foam everywhere _____ you remember to engage the magnetic cup holder.

The correct answer is unless.

"Unless" means "if not" or "except if." The sentence means the machine will explode if you do not remember the cup holder. Using "if," "provided that," or "as long as" would hilariously imply that the machine explodes when you do everything right!