[newbie]substitution - square brackets

I am trying to learn Tcl with the help of TclTutor. I seemed to
understand substitutions, but, when playing around with the examples, I
tried out this:

set foo [puts test]

and I did not understand what the interpreter returned (a void string).
I would expect to see the variable $foo set to "test".

As you can you guess, I'm a very beginner.
Hope you can help me,

FS
Received on Sun Apr 30 02:31:04 2006