A Spotless Mind

On the elevator with me are a mother (M) and son (S) on their way to school. The door slides open a few floors down and a father (F) and daughter (D) step on with an elderly lady (EL) who is unrelated. This is what I overhear:

EL to D: Are you off to preschool?
D: Yes.
EL: That sounds like fun! (turning to S) And are you going to preschool too?
S: Yes.
D to F: I know him (pointing at S)!
F: I know.
M to D: Are you in the same school?!
D: No, but we live in the same building.