Learn Finnish: Minä käyn vessassa.

In English, you say you 'go to' the toilet. But the Finnish verb käydä (to visit / to go and return) requires you to use a static 'where' location ending, like the inessive -ssa (in). You are literally saying 'I visit in the toilet.'

How is käydä different from mennä (to go)?

Mennä means going somewhere in one direction, and it takes a destination ending like vessaan (into the toilet). Käydä means making a round trip: going somewhere, spending a little time there, and coming back. Because the focus is on being at the location during the visit, it uses static cases.

Where does the word vessa come from?

It is the most common and natural word for toilet or bathroom in everyday Finnish. It originally comes from the letters 'WC' (water closet). Finns pronounced the letters as 'vee-see', which eventually turned into the everyday word vessa.

Do I have to include the word minä?

No, you can just say Käyn vessassa. Because the verb ending -n already tells us it is 'I' doing the action, the pronoun minä is optional and often dropped in everyday speech.