Why do they number the floors differently?

I’m hanging out in Budapest for the weekend. While in the hotel elevator, I overheard this American couple asking the same stupid question that every American that leaves her country for the first time asks:

Why do they number the floors differently?

After I resisted the urge to scream and explain the concept of cultural and linguistic differences to them, I decided to write a short post with the actual boring explanation:

In most European languages, the word for floor does not mean the same exact thing. Instead the French word etage or the German Stock actually mean something like added level. So of course the first added level is the not the ground level but the one above.

So simple and yet it has challenged travellers for ages.