xmas lights

There are Xmas lights decorated to a tall building

What is the preposition that I would use?
Do I say "watch the lights on the building"
Do I say "watch the lights in the building"
Do I say "watch the lights at the building"

[Soup]

... on the building.

[rancher247]

Hi! I am not a teacher, but I am a native speaker who is in college in the United States.
I'm assuming that you meant that the building is decorated by christmas lights on the outside, and that "you" are telling someone to look at them instead of "watch" them because one generally does not stare at the christmas lights on buildings, looking at them blinking or something. One of the important distinctions between "look" and "watch" is that watching implies that what you are seeing is moving. For example, "I'm going to go watch television." Sometimes people use "see" when they mean "watch"-one instance that comes to mind is "Hey, do you want to go see that movie with me?" This is normal and okay, but it's a more general term than "watch".
If that's what you meant, you would probably want to say "Look at the christmas lights on the building!" (probably followed by a sentence like: aren't they pretty? don't you love them? I love the christmas season....and so forth.)
Please do not hesitate to correct me or ask me any questions you may have about my answer to your quandry.