It's just a rather idiomatic phrase that many teachers use. Students seem to grasp the intended meaning quickly.
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RommelI wonder, “How could one possibly erase the board?”Then you must also wonder a great many other things, like "How could one possibly open an envelope?" Don't we open a door to enter a room? We don't "open the room". So in that case, don't we open the flap (of the envelope) to enter the envelope?