i'm surprised if this isn't a repost, but i searched to no avail. i don't think i should put this under games since it's more of a brain teaser type thing. it took me a few minutes, but it's interesting to see what hypotheses people come up with for the solutions.
if you figure it out, pretty please don't tell how and spoil it for everyone else. half the fun is watching people struggle and like some of my friends have only to laugh once they know the method. don't think too hard and enjoy