Hold up both hands and number them 1 through 10, left to right. Want to work out 9×4? Fold down your 4th finger. Now there are 3 fingers left standing to the left of the folded one, and 6 to the right. Put them together and you get 36 — which really is 9×4! Try it with any number and this trick always works out exactly right. It looks like magic, but a very simple piece of math is actually hiding underneath it.
The secret is in the fact that 9 = 10 - 1. Working out 9×N is exactly the same as working out 10×N and then subtracting N once (9×N = 10×N - N). For example, 9×4 equals 10×4 (=40) minus 4, which is 36. The finger trick is simply a way of showing this "multiply by 10, then subtract one" process with your own hands.
Why does the left side of the folded finger become the tens digit? Fold down the Nth finger, and (N-1) fingers remain standing to the left. That number becomes the answer's tens digit. And to the right, (10-N) fingers remain, and that becomes the ones digit. Add the two numbers together: (N-1) + (10-N) = 9. It always comes out to 9. The reason the tens and ones digits always add up to 9 is, in fact, exactly the same story as the well-known property that the digits of every answer in the times-9 table (9, 18, 27, 36...) always add up to 9.
What makes this trick fun is that it's not "a formula you memorized" — it's "a principle you can check for yourself." Even if you haven't fully memorized your times-9 table yet, your fingers alone let you instantly work out anything from 9×1 to 9×10. And once you understand the underlying reason it works, memorizing your multiplication tables later will feel a lot more familiar too.
This kind of "finger counting method" is actually one of many mental-math techniques handed down for a long time in cultures around the world. Before calculators existed, people came up with all kinds of ways to make calculation easier using their own bodies. There's a method for multiplying two-digit numbers with your fingers too, beyond just the times-9 table — and just like what you learned today, that one can also be explained mathematically, in terms of exactly why it works.
On our activity page, you can pick which finger to fold with a slider yourself, and see with your own eyes that the number of fingers left on the left and right exactly matches the real multiplication answer. Try it with several different numbers and check whether the trick really does always work.