In Through the Out Door (Led Zeppelin) Easter Egg - Inner Album Jacket

On the inside album cover (hard cardboard), there
is an outline drawing on either side. One one
side, there's a drink, a pack of cigarettes, a
few almonds, money, a clean ashtray, etc. On the
other side, the drink is half gone, the nuts are
cracked, the cigarettes are smoked, there's change
on the bar, etc. Take a damp cloth or a sponge
and rub it across either side, and as you do,
color comes out on all the items.
Warning: this is not reversible. I recommend that
you do it just on one side, so you can show people
later (better yet, just 'color' one item at a
time, and you can show the effect to numerous
people).