the answer to your second question..lsd, ecxtasy, any mind enhancing drugs :)
the answer to your first...we all grow out of a single cell...it divides once, then divides again, etc. at the beginning stages, what each cell becomes(nervous system, muscles, etc) is not exactly determined and can be changed. when you have a blastula(which is what the ball of baby cells is called) you can pick and choose what the cell becomes later on in it's life. so stem cells are good for producing whatever the hell tissue you want.
stem cell therapy is when you put this newly formed tissue in a human body rather than in a petri dish - so that you can put stem cells in a damaged heart(eg, vicitims of myocardial infraction, er, heart attack) and force them to become healthy heart tissue! amazing stuff is going on here...seriously...or a stroke victim and force the tissue to become new neurons...crazy ridiculous amazing science and bush is fucking it all up!
stem cell therapy - good website:
http://www.stemcellresearchfoundation.org/About/FAQ.htmhope that helps!
captjack