Sprint the length of the gym, or some other set distance. Easy, right? Now do it twice Now do it 3 times Now do it 4 times Now do it 5 times Now do it 6 times Now do it 7 times Now do it 6 times Now do it 5 times Now do it 4 times Now do it 3 times Now do it twice Now do it once I nearly passed out after doing this one day because the gym teacher hated us.