Disappearing Ink

This ink-credible Disappearing Ink Pen Tube is full of a blue
coloured liquid that will vanish before your eyes! You'll have
your friends and family asking, "Is it magic or is it science?".
The secret is in the chemicals that make up the ink. When you
write with or squeeze this fluid onto something like a friend's
t-shirt or school assignment, the chemicals in the blue ink react
with carbon dioxide in the air as the liquid dries to make the
ink disappear. Great prank for April Fool's Day or a magic show.
Leaves no mark or stain. Avoid contact with eyes.