Sweets has asked them to reflect upon the things that hold their partnership - their friendship - together. He thinks it will strengthen their bond and reduce their bickering, which has increased as of late.
Booth, of course, balks at the idea of having an essay assignment so Sweets mollifies him with simply making a list.
Oh, like a top ten list?
Right, Sweets agrees encouragingly.
Brennan objects to the number. Why must there be a specific number? There should be as many as are appropriate. It could be six or eleven.
Booth argues that it is nice to have a round, even number.
Brennan interjects with nine; nine is a better number, then, because it is a perfect square.
Booth scowls and Sweets convinces him to compromise with nine, which he does, but Sweets smiles as he can see that Booth will still come up with ten.