A couple of years ago, my wife bought my then four-year-old son a supercool set of wooden ramps, which could be combined with our furniture to create courses through which little balls could run. Building the first course was easy, but, as our ambitions grew, the difficulty level rose. Could we make the balls turn corners? What about generating enough momentum for them to go briefly uphill? Would the ottoman support a ramp? What about the piano bench? The variations seemed endless. Two or three ...