Oh man why me?

We were at the store the other day waiting in line when my 4 year old looks at the lady in front of us and asks me ( in a very loud voice) " Mom do you like that ladies hair?"   I quietly tell him to hush and then he says "Man how did she get her hair so high, it looks like a tangled birds nest!"    Ummm what do you say to that....   I was so embarrassed, for one she heard him, two her hair looked exactly as he described it and three the people standing in back of us were laughing.   How do you teach a 4 year old to bite his tongue???