Caitlin arrives with the pizza, but Ross screws up the flirting again, ending up talking to her about gas and methane. He scares her so much that she pays for the pizza herself and flees the apartment.
When Caitlin brings pizza, Ross fails to impress her romantically, instead engaging in a discussion about gas and methane. His awkwardness causes her distress, leading her to pay for the pizza and flee the apartment.