From Wikipedia's page for the author: http://en.wikipedia.org/wiki/William_P%C3%A8ne_du_Bois

Peter Graves (1950): A well-meaning but mischievous boy who encounters a gentlemanly and not-very-mad scientist named Houghton Furlong. Furlong is the inventor of an antigravity material named Furloy, and a Furloy-based invention called "the ball that bounces higher than the height from which you drop it." In an unfortunate accident with the latter invention, Peter destroys Houghton's house. Little of value is left in the wreckage except six balls of Furloy, each about the size of a tennis ball, with an antigravity pull of 25 pounds-force (110 newtons) each. Peter commits himself to spending the summer with Houghton in an attempt to earn the $45,000 necessary to rebuild his house. The implied puzzle is: how can Peter and Houghton make use of the six Furloy balls to earn $45,000?