If you want to have real fun with watermelon, eat a good amount of dried apricots or prunes, then eat half a ripe watermelon (no salt); stay near a bathroom for the rest of the afternoon........ then take charcoal.
Wow I really should have taken this advice more serously! Ate a whole lunch box of water melon in The Car and damn near didnt make it home before I exploded (Sorry for The TMI).. Does it work through serotonin u Think?