you'll need = a standard dishwashing sponge (others might not work as well, I tried). = a bic pen (or another pen with a somewhat bendy/soft plastic shaft) = a metal rod (I got mine from a broken umbrella of which there are plenty in nyc) = a cigarette lighter or maybe matches.
1) pull off the tip of the pen (unscrew it, yank it, torque it, whatever) take out its insides so that all you have left is the tube and the tip
2) using the lighter heat up the end of your metal rod (maybe hold the rod with paper or something so you don't get burnt)
3) shove the hot tip of your metal rod through the plastic near where the tip goes on. shove it in so that the metal rod sticks into the area that the pen tip will cover when you put it back on.
4) put the pen tip back on (it should have a hole instead of an ink thingy at the tip at this point)
5) shove as big a piece of sponge as you can into the tip. If a lot hangs out, snip it off with scissors.