The ancient Incas are said to have communicated by "quipu"--knotted strings which encoded information. You too can encode information in a simple knot. If you knit a swatch on size 7 needles, 7 knots go in the tail. The next time you wonder what needles you used with that yarn--ta da! Your own personal "quipu" is hanging on the swatch waiting to tell you. If you "borrow" your best size 5 circular needles from a project and put the project on a stitch holder, 5 knots go in the tail. When you find that project languishing 6 months from now, you won't be discouraged from taking it up again by not remembering what you were doing--your 5 knots will remind you that you were knitting with size 5 needles. As is apparent, this also works with millimeter sizes BUT you must not mix millimeter and American sizes--pick one or the other.