Basic tower marble track
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Marble game
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Put a marble in the top, somewhere where there's a hole or gutter, and it will come out at the bottom at either of two ends, somewhat randomly. I got lucky here, the marble and Duplo sizes and the gravitational constant on Earth in this universe come together to create this 'perfect storm' where the marble doesn't always end up at the opposite, or the same side. Which makes for a lot of laughs from my kids. (The 4 year old understands the idea easily already, the 3 year old seems to not notice yet but laughs anyway.) The 'fun generator' in full scantily clad glory:
My 3 year old made some awesome art and explained it to me, which helps me in adding captions.
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A while back I made a different marble game. A wall of Duplo with various sized holes in the bottom, for the kids to try to roll the marble through. They have difficulty keeping attention but it does work and they get it. No pictures though but the idea is more obvious.
A was hoping I could activate the Duplo siren lights I got by letting the ball roll against the button, but it doesn't budge And Duplo lacks the means of implementing a reliable lever as a force multiplier.