For a tiny distribution, puppy or slitaz work quite well.
Also, I would say that Arch Linux can be as tiny as you want, but it's not a fire and forget solution and I wouldn't want to maintain an Arch system for friends or relatives ;)
Uh yeah probably shouldn't jump into the tiny Linux debate, but I recommend looking into Rich Shingledecker's attitude towards desktop distribution bloat even on versions considered "tiny" vs the 6 to 12 MB distro that he turned into Core Linux. Divergent philosophies, and I found it best to leave the desktop world behind when approaching Linux education. Best not to confuse what's being worked on to compete with iOS, Android, OS X and Windows with the education that will serve you best on servers and embedded systems. Even Puppy and SliTaz are large-ish desktop versions of Linux in comparison.
Also, I would say that Arch Linux can be as tiny as you want, but it's not a fire and forget solution and I wouldn't want to maintain an Arch system for friends or relatives ;)