i'm still sort of surprised this worked!
to make these secret message ice cream floats for olivia's ghostwriter birthday party, we first wrote a message on clear plastic cups with a turquoise permanent marker. then using a red marker we drew a series of x's over the message to obscure/encrypt it. (i like how the result is sort of an argyle pattern.) at the end of the party we revealed the secret message ("happy birthday olivia") by filling each cup with red soda + a big scoop of vanilla ice cream.
[click the photos above to enlarge them + see the effect more clearly.]
for another execution of the same effect, see the party invitations here.