Just a quick post (because I don't feel like getting out my camera).
If you are making socks (knitted or crocheted) and want to reinforce the heel and toe, try using matching polyester sewing thread. It comes in far more colors than reinforcing thread and won't change your gauge.
But when you do use that sewing thread, you will need to keep it contained in order to prevent nasty tangles with your yarn! Seal the spool in a zip lock bag and feed the thread out of a hole snipped in a bottom corner.