Knot Trick

Materials needed:
A string or rope about 20 inches long.

Place the string on a table and challenge your friends to hold
one end of the string in each hand and tie a knot — without letting
go of the string. No matter how hard they try, they will not
be able to tie a knot without releasing at least one hand.

How to do it: Cross your arms, bend over the string, and grab
the ends in your hands. Uncross your arms without letting go of
the rope, and you will have tied an overhand knot.