Ok! first of all, tell the spectator to think of a number between 1 -50. tell him / her to use odd digit numbers
for each numbers but use defferent digit numbers. The spectator should be thinking of the number 37 because it's odd and most
of the people will think of the same number because it's commonly been used.
Remember! it doesn't works all the time and if it is so, it might work by properbility.