A Boy's Best Friend
As far as I know, this was the shortest robot story Asimov wrote. It's only 4 pages long, but it's also one of his most poignant stories. It is basically a cut-down version of the story Robbie, which Asimov wrote 35 years beforehand, and is about a boy and his robotic dog. His father wants to replace the dog with a real one, but the boy does not see the difference and wants to keep his robotic pet.