Find min and max values across two columns?

Hello. I've recently begun to use PostgreSQL in earnest (working with
data as opposed to just having clever applications tuck it away in
there) and have hit a wall with something.

I'm trying to build a query that among other things, returns the
minimum and maximum values contained in either of two columns. The
problem is that min() and max() only take one column name as an
argument. Is there some clever way that I can craft an expression to
find the min/max across both columns? I have yet to delve into any
pgsql so if it requires that, then please go easy on me. :)

Example:

id | columnA | columnB
1 | 4 | 2
2 | 3 | 4
2 | 5 | 1

I'd like to be able to discover that 1 is the smallest value in
either columnA or columnB and 5 is the largest. I don't actually care
what rows they are in, I just want the values.

Thanks for your time! Any help or pointers to relevant reading
material on this would be greatly appreciated. (I have been using the
excellent PostgreSQL docs and an equally good book titled Beginning
Databases with PostgreSQL by Neil Matthew and Richard Stones so far.)

--
Amos