Listen to the song. Use this as a stimulus for the children to understand what subtraction means. Get the children to write down as many words for subtraction as they can. They can also use this as a memory exercise, where they write on their whiteboards as many different subtraction facts as they can that feature in the clip. From this they can then use a variety of everyday objects and apparatus to create a variety of subtraction calculations (with their inverse) by practically working these out, eg 20 - 5 = 15, 15 + 5 = 20. This will help reinforce their number bonds too. To stretch the more able, you can use this song to help find the missing number in the number sentence eg 5 + ___ = 21?