Rochester Memorial School students were treated on Tuesday afternoon to an educational exhibition of the dog-like robot "Spot," a $200,000 machine of which there are approximately 1,500 in the world, according to Kevin Garell, a Rochester resident who works for Waltham-based Boston Dynamics, the manufacturer of the robot. Garell was assisted in two presentations by his sons Gavin and Grady. Photos by Mick Colageo