http://www.englisch-hilfen.de/en/exercises/modals/can4.htm
http://www.englisch-hilfen.de/en/exercises/modals/can3.htm
http://www.focus.olsztyn.pl/en-can-cannot-exercises.html#.WLVQZjs1-7R
http://spot.pcc.edu/map/prev/can-cant.htm
http://www.englishexercises.org/makeagame/viewgame.asp?id=1518