Polypropylene pellets as an inexpensive reusable substitute for milk in the Morris milk maze

A new inexpensive opaquing agent for the Morris milk maze is described. Small light-weight polypropylene pellets that float on the surface of the water were used to eliminate visual cues about the location of the hidden platform without impeding swimming or the use of distal spatial cues. Results obtained using the pellets are identical to those obtained with milk powder as an opaquing agent. An automatic tracking system works as well with the pellet as with the milk version of the maze.