To get the full effect of this conversation, you need to imagine Derek’s most enthusiastic dad-voice.
D-Who’s ready to go ride bikes? I am!!
L-I am!!
D-Who has their shoes on? I do!!
L-I do!!
D-Who’s excited? Me!!
L-Me!!
D-Who’s the biggest dirtbag??
L-DADDY!
He is one smart cookie. He clearly understands that D is the biggest dirtbag in this family. 
