Explaining "As much use as a chocolate fireguard" to an adult student:
"So a fireguard is a little iron wall, and you put it in front of the fire for safety. So the children can't get near."
-"Aaah, yes, I know.
"So, how useful is a chocolate fireguard?"
"Good! And why not?" (I want to see if he knows the word "melt.")
-"It is not useful! The children will eat it!"