Who belong to Ahle Bayt?

I was always under the impression that the Prophet's (sas) family is considered Ahle Bayt ("People of the House") which means His wives and children, but I found that my Shia friends consider the Ahle Bayt only 1. Hazrat Fatimah (r.a.) 2. Hazrat Ali (r.a.) 3. Hazrat Hassan (r.a.) 4. Hazrat Hussain (r.a.) I asked them: Why do you consider the above persons only as the Ahle Bayt? Why do you not consider the Prophet's (sas) other daughter's and their children as Ahle Bayt? They said that the Prophet (sas) brought only these 4 persons under his cloak and called them his Ahle Bayt. Then I asked them: Why do you not consider the children of Hazrat Hassan's (r.a.) children as Ahle Bayt? But they did not have an answer. I think all the children of the Prophet (sas) are Ahle Bayt.