No calculator, no memorized times tables — just fold down one finger and the answer to a times-9 multiplication appears instantly. Use the slider to pick which finger to fold, and check that it's really correct.
Hold up both hands and number them 1 through 10, left to right. To multiply 9 by some number, count from the left and fold down that many fingers. The number of fingers left standing to the left of the folded one becomes the tens digit, and the number left standing to the right becomes the ones digit!
This trick works because 9 times any number always equals 10 times that number, minus that number. The one folded finger represents "one taken away from 10," which is exactly why simply counting fingers hands you the right answer.
Before calculators existed, hand-counting tricks like this were genuinely useful in everyday life. Change the number you're multiplying by with the slider, and check for yourself, one at a time, whether the finger counts on either side really do give you the answer.