When Steve and Shivani arrive at Green Park station they discover that, much like certain cakes, the toilet is a lie.

S1E13 - Victoria Station

Steve and Shivani used to talk about mundane, everyday things like cats, the weather, or quantum mechanics. But now, they've turned their attention to far more interesting and cultured affairs: Publicly accessible restrooms. They used to visit a different toilet every week and discuss their experiences, but that got boring, so now they're just talking about whatever weird and wonderful toilet stuff they can come up with.