Is too.

My kids are usually good for a laugh, but this morning they were in rare form. Brandon had said something that I didn't hear and all of the sudden it was:

S: "Is not!"
B: "Is too!"
S: "Is not!"
B: "Is too!"
S: "Is not!"
B: "Is too!"
S: "Is not!"
B: "Is too!"

I asked Stacey what was going on and she said, "Brandon keeps saying stuff."
I couldn't help but laugh.