Built for small hands. Every tier is a solid block with a flat bottom and a ramped top — no thin shelves — and stacks on a keyed plug that only goes together one way (a parent can pull it apart; a toddler can't). The balls are 45 mm, well above the 31.7 mm small-parts test, printed as two halves that we glue into one; nothing on the tower is smaller than the column. Ages 12 months and up; PLA, wipe clean. Extra balls are $4 each — pick "Extra ball" above to add some in their own colors.