Any time Renton gets überexcited because of something Eureka does obliviously. Such as, once they were doing the washing up, and Eureka got a little closer and their were in contact. "Right now, all my nerves are concentrated in my arm! Or rather, my entire self is my arm!" with an incredibly pleased face.
Eureka "wearing" makeup straddles the line between this and horrific. No, really.