This. From what I've heard the yanks at customs are a real pain in the arse. Better to be safe.
Real pain in the ass. Last time I flew into Boston our conversation went something like this.
Customs : Purpose of visit
Me : Holiday. Visiting my sister.
Customs : *peers at me* I don't understand.
Me: ..... What don't you understand ?
Customs : What you are doing here.
Me : Visiting my sister.
Customs : Why?
*by now I think he is either taking the **** or just awfully stupid*
Me : I would like to see her again.
Customs : I'm not following.
Me : What don't you follow?
Customs : You haven't told me what you're doing here.

.....
And so it went of for a minute or two until he got bored and handed me back my passport and waived me in with a sigh and snort.