A B-tree is how databases keep millions of sorted keys shallow enough to reach in three hops. Nodes hold up to 3 keys here — the 4th forces a split, and the middle key gets promoted to the parent. Watch it happen, key by key.
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Enter inserts the key in the box. Try 43, then 45 — the second one overfills a leaf and splits it. Click any key in the tree to load it into the box, then Delete.