Make a set of 40 rectangles of card approx. 6cm x 2cm.

Colour them red with a silver end, to look like bar magnets. On the silver end label half with N and half with S. On the red ends put the numbers 1 to 20 (one per magnet) so you have two sets of magnets nymbered 1-20, half of them N and half S.

The children put the 40 cards face down and take turn to pick up two. The aim is to find a number pair that add up to 20, one must be N and one S, reinforcing the principle that unlike poles attract.