The teaching aid apparatus provides two balance beams (27, 28), one above the other, on a support post (23). Weight-attachment stations (36) are provided on the left and right arms of both beams, and the stations are marked on the beams with numerals indicative of distance of the station from the beam fulcrum. The beams are connected by means of pinions (32, 34), which constrain the beams to tip in unison in response to unbalance of one beam. The apparatus aids the demonstration of negative concepts to young children.