Office Building?

This morning on our way to Evan’s daycare, we were passing by a series of brown office complex about 4 stories high for Research and HR of my company (the daycare is right outside the company) when Evan asked me:

“Mommy, what is that?” he pointed at the office complex.

“Office building. 大楼, I told you before.”

“No, that is not大楼, that is a city!”