Arithmetic is the death of story.

Dialogue is generally the worst choice for exposition. When you're writing lines you need to focus on the way people actually talk. And when we talk to each other we never actually explain our terms. We don't say 'Sweetheart, would you pass me the sugar bowl, which we picked up for a song at that antique stall in Munich.'

Just start the sentence...and see what happens. This is how we write.

That's the hard work of writing. The imagining.

You might ask yourself why you want to surprise your readers in the first place. A surprise ending is sort of like a surprise party. Probably some people, somewhere, enjoy having friends and trusted colleagues lunge at them in the sudden blinding light of their own living room, but I don't think most of us do.