Digit span test: how many digits can you hold?
Glance at a string of digits, then type it back. Each time you get it right, the next one is a digit longer. Miss twice at the same length and that's your span. No lesson, nothing to learn first - use whatever tricks you already have.
What digit span measures.
Psychologists have used this test for over a century. The classic finding is that most adults hold about seven digits, give or take two (Miller, 1956). Later work argues the real limit is closer to four chunks, and that people reach seven by quietly grouping digits together (Cowan, 2001). Grouping the first few digits as a shape and saying the rest over to yourself is exactly that kind of grouping, and it's why the same person can score differently from one try to the next.
This version isn't the lab one. The clinical test reads digits aloud, one per second; here you see them all at once for a few seconds. So your score is a game score - fun to compare with yourself, not a diagnosis.
This isn't what Memory Mill teaches.
Memory Mill teaches memory techniques - turning digits into pictures and stories. They don't enlarge short-term memory - they work around it, by giving digits something to be. If you're curious what that looks like, the home page test gives you twelve digits twice, once plain and once as a story. This page is just the game.