function add (first) {
    return function (second) {
        return first + second;
    }

@Douglas:
I'd also appreciate more puzzles if you know any. This kind of (simple
but interesting) problem is a godsend when you're teaching kids.

The only thing I've seen recently that comes close to a puzzle is this
website:

  http://parentnode.org/static/challange2.html

You need to enter the correct password, that's all. No rewards :-) It's
not pretty, rather the opposite, but if you like obfuscation, it might
be interesting. Syntax highlighting helps.