Contractions

Student #1:  "I ain't doing that."
Student #2:  "Ain't isn't a word."
Student #1:  "Yes it is.  It's in the dictionary!"
Me:  "Yea, the ebonics dictionary."
(students ignoring me)
Student #1:  "Yea...ain't is a word!"
Student #2:    "Okay, so can't is a mix of can and not.  What is ain't a mix of?"
(Student #1 thinking)
Student #1:  "Ain't and not!"
Student #2:  "That doesn't make any sense."
Student #1:  "Yea, I messed up.  Ain't is a mix of: and it not."
(silence)
Student #2:  "Wow."