sparkfun Simon Says Assembly Manual page 6

The next bit of advice is always keep your iron in its stand while your not using it. Setting down your iron on your
workspace can be a major hazard. Not only can you burn the other items on your workbench, but you can also set
your arm down on it and get a gnarly burn (I speak from experience). Your iron has a home, and we're all better off
when you keep it there.
Maintaining Your Soldering Iron
You'll notice that on your stand there is a little pool with a sponge in it. This sponge plays a very important role in
soldering. As you solder, old solder tends to clump up and loose it's ability to melt and flow properly. It can also
build up a nasty residue on your iron tip and corrode it to the point were it won't accept solder any more. That's
where the sponge comes in handy.
Dampen it in a sink or using a water bottle. It doesn't need to be soaked, just wet enough to pull solder off you iron.
Now, very gently, wipe the excess solder off onto the sponge. Avoid jamming the tip into the sponge or leaving the
tip on the sponge for more than a few seconds. Doing so could result in holes in you tip, which ultimately ruin your
iron. You should periodically clean your tip as you're soldering. This will make your solder connections look nicer
and will increase the life of your iron. To increase the life of your iron even more, we recommend using a brass
sponge instead of a regular one. Expansion and contraction of the tip, from the cool water, tends to wear it out
over time.