Do you want to improve your understanding of when to use an apostrophe?  Playing these games will help:

http://www.bbc.co.uk/skillswise/words/grammar/punctuation/apostrophes/game.shtml

http://www.quia.com/jg/86241.html

http://www.ngfl-cymru.org.uk/vtc/apostrophes_con_poss/eng/Introduction/default.htm - Click the starter and main activities and you will go through some teaching and learning activities.

http://www.oswego.org/ocsd-web/match/dragflip.asp?filename=jwildecontraction2