Our Expanding Universe

I have always wanted an answer that I can understand to the following question.

Simply asked, "What is our Universe expannning into"?

Please explain it assuming I am an *ordinary* 13 years old.

I finally found this question and its answer, in a FAQ called
"Frequently Asked Questions in Cosmology":

http://www.astro.ucla.edu/~wright/cosmology_faq.html

It is all "%^$@^$%@^" to me. :-(

Thanks in advance to "anyone/everyone" who can explain it to
me so that I understand.

BluMax