What's Your Last Name?

A conversation between Liam(5 yrs old) and Aidan (7 yrs old) at the dinner table last night was amusing but left me questioning

Liam : Do you know what they call me at school ( he's in preschool)? They call me Liam R-

Aidan: Yeah R- is your last name.

Liam: Yeah, Aidan what's your last name?

Aidan: My name is R- too, this whole family's last name is R-

Liam (somewhat surprised): Oh yeah, your last name is R- too.

Ummm, hello, knock, knock, anyone upstairs there. How is it that he doesn't know our family's last name is R- ? So we had a discussion about how we all had a different first and middle name but how we all had the same last name. Lets hope he gets it now.