I was curios about it too. Straight googling was fruitless but guessing at the solution (Something within the common functionality of a truck and which would be "obvious" in hindsight and thus work for a riddle.) The google string that worked brought this up: http://rawksoup.com/neighborhood/pg/blog/read/1987/brain-tea...

There's a widely circulated story about everyone being stumped about how dislodge a truck wedged underneath an overpass, then a child suggesting that they let air out of the tires. It may be apocryphal, but it used to be referenced pretty often.