[plt-scheme] Syntax question - begin in threads

I've been experimenting with threads, and find that this code works
just fine

(define x 0)
(define t1
  (thread
   (lambda ()
     (begin
       (sleep 5)
       (set! x 10)))))

but, if I leave the begin out, I get an error (though the thread runs).
I thought of begin as a *sequencing* command. What if I have several
expressions that need to be evaluated, but I don't care about the
order.

Am I misunderstanding the role of begin?

===
Gregory Woodhouse  <gregory.woodhouse at sbcglobal.net>
"If you give someone Fortran, he has Fortran.
If you give someone Lisp, he has any language he pleases."
--Guy L. Steele, Jr.