Built around the number five

The rekenrek, or arithmetic rack, is built entirely around five: each row holds five red beads and five white, so students can "see" seven as "five and two more" without counting. That five- and ten-structure is the backbone of early addition and subtraction strategies — making ten, thinking in fives, knowing how far a number is from ten. This tool flashes bead configurations briefly so students subitize the arrangement and reason from its structure, then slides the beads in on the reveal exactly as they'd move on a physical rack. Because beads on a rod can't pass each other, the tool only ever shows arrangements a real rekenrek can make, keeping the on-screen model honest. Flash a configuration, give think time, and ask "How did the red beads help?" — the five-structure students notice is the whole point.