With either hand, place the tip of your forefinger to the tip of the thumb on the same hand, thus forming a hole.
The question is, can you poke your head through this hole (without losing contact between the finger & thumb tips)? If you can't, you might want to consider another trade.
For those of you that know the answer, please keep it to yourselves for a little while.
Few things are harder to put up with than the annoyance of a good example.