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Inside the toilet?

Well, "am" is not a prepositon, Gorky. It's the first person singular of the present tense of the verb "to be". :?

It's not necessarily "cooked in", as you say. Я сижу can be non-continous too.

Я сижу на стуле.
Я сижу на скамье.

It's because you sit on some surface in Russian.

Trickiness is also here, when we comapre two languages:

Я сижу на дереве.
I sit in the tree.

Prepositions are tricky in EVERY language.