A prime number is a number that can only be divided by one or itself.  It is important your child knows their prime numbers off-by-heart for their 11+ as the concept is very likely to arise.  This game will help them to have fun while doing this!  Cut up some card so that you have 25 small squares.  Write the numbers 1 to 25 on these.  Put all the cards face down on a table.  You will also need a basket or pot to 'throw away' numbers that are not prime.  Take it in turns to turn over a card - if the number you turn over is prime, you keep it, if it is not you 'throw it away' in the basket.  The game ends when all the cards have been turned over and the winner is the person with the most prime numbers.  For more ideas on prime numbers: https://www.pinterest.com/search/pins/?q=prime+numbers