My nephew is Californian and I gave him one for his sixth birthday. Mine felt a bit lighter so we did an experiment over Zoom together to measure the density. He’s certainly a bit young for that — “it’s the same size but yours is heavier” — but there’s no harm in influencing them from an early age.

Of relevance to your comment: I made a point of doing it in grams and millimetres.

How large is one of those, even? I much prefer to work with perfectly spherical Olympic swimming pools.