AustLit

'A boy is given underpants that, despite their look that would cause him to be teased by the school bully, give him superhuman strength. But when he takes a break during a cross-country race (he is first by a wide margin), those underpants start to shrink, and the bully eventually comes to steal the rest of his clothes. He ends up going home in the nude, causing him to be grounded, but he eventually enters a mouse race in which the winner gets 50 dollars. The boy wins the mouse race by giving the mouse the pants. Was later adapted as an episode of Round The Twist.' (https://en.wikipedia.org/wiki/Unreal_(short_story_collection)