Get pairs of socks/shoes and count how many altogether.Show that you can count them in twos to make it quicker to count. I also like to use a number square to show that I am counting every second number. This helps the children to understand what they are doing. Likewise when we count in 5s and 10s I show them it on the number square. Children can then have a paper number square and highlight the 2s numbers. Whenever we go on visits and the children are lined up in pairs I always count them in 2s out loud so that they see a context for counting in 2s.