Being a geeky type, I have a preferred writing tool. It is a Staedtler Mars Medium mechanical pencil, in 0.7mm, and I prefer to feed it 2B leads for extra legibility in its primary usage, which is completing the cryptic crosswords that one C. A. J. was kind enough to send me as a gift around the time the LA and I married. While said crosswords are distinctly fun, the paper upon which they're printed is rather cheap, and rapidly fades to yellow, making HB marks illegible. Note for the US/Canada readers: HB corresponds to a #2 pencil, while B, 2B and so on are softer ("Black"). Meanwhile, H, 2H and so on are harder ("Hard"). The difference is ratios of graphite and china clay, and the more china clay the harder the pencil and the lighter the line. My preferred hardness is a little softer than usual, and so is less than common, particularly in 0.7mm, since the standard for mechanical pencils is 0.5mm. Unfortunately, 0.5mm tends to poke holes in my crosswords.
Fortunately, tonight the LA and I passed through Staples, and found 0.7mm B leads. Not quite my preferred 2B, but perfectly acceptable. I can return to smooth writing and legibility in my crosswords, and cease worrying, since they were in packs of 36 leads. Since each lead is supposed to write for as long as a standard pencil, this is remarkably good value... you try finding 36 B pencils for less than $2.50!