Building sites is dangerous

There is a building site at my work (show me a university that doesn't have at least one building site on campus at any given moment) and it has this sign:

'Building sites are dangerous; Keep out'

Every time I walk past it (a couple of times a day at least) I think to myself 'Building sites is dangerous'. This is a sort of in-joke that me and approximately one other person in the world will find amusing, and he probably doesn't read my blog, so I'm going to explain it to you all instead and tell you about the moment I decided to become a syntactician.

It wasn't exactly the moment I decided to do it as my job, but it was the point of no return. It was in my very first syntax lecture, in September 2004, when aged 21 I had decided to go to university to do Linguistics. Why linguistics? Not sure. I don't think I was specially bothered about English Language in 6th form, but I did do three languages and enjoyed the grammar. (That's why I signed up to do Latin as my optional outside subject, and it's why I'm currently learning German.)

Anyway, back to the syntax lecture. The lecturer was Noel Burton-Roberts. I can't remember what else he said in that lecture (I've got the notes so I can look it up) but he used an example to show that syntax is a thing, and that words aren't just strung together in order.

Flying planes are dangerous (=planes that are flying are dangerous)
Flying planes is dangerous (=the activity, flying planes, is dangerous)

The verb has to agree in number (be singular or plural) with the subject of the sentence, and specifically the head of the subject. The subject of that sentence is flying planes both times, but in the first instance flying describes planes, so planes (plural) is the head and we have are, while in the second example it's flying that's the head, and planes could be left out (flying is dangerous), and flying is singular so we get is.

Such as simple example, but upon seeing this something just clicked in my mind. It's a cliché, but it's true. It was as if I had suddenly discovered this whole secret world, where the language we speak every day had proper structure and rules and could be explained. Everything from that point on just made sense.

I suppose that makes me a bit weird. But it's why I'm a syntactician now.