Thank you everyone! I've suddenly got a head full of ideas I want to play around with - so exciting!
Katxena - my felt tip pens have a cylindrical sponge thing inside them that holds the ink - I guess it's a way of having an ink reservoir that doesn't slosh around... you can get the sponge thing out by popping the end of the pen off. Ummm, here's a crappy pic:
The green pen on the right I've opened up - the pink one is non-dismantled for comparison.
Hope that makes sense!