My kids asked me this and I have been stumped.  I have asked several sources and no body answers or can give me a good answer.
133441
And here I thought I come up with weird Questions...You win...#1 and #2
489517
I know the answer is in my basic physics book.  I would have to look it up.  You should too.
168255
Extremely difficult question to answer for me because I am not a scientitian. I mean a scientist.
121392
What class was this in? What grade level? What text were you using?  I may be able to help you if I know the context in which the question was raised. I have a feeling that I am not getting all the information you had. Please tell me and maybe I can help. I am an old chemist and microbiologist.
270267
Sorry kid...you have to do a little research on your own sometimes.  Try wikipedia.
546115
I asked that question, and there is no known mechanism. There is however a mechanism that keeps different species from being able to interbreed.
87328
this sounds like a homework assignment~ i suggest that noone answer this and have this person DO THEIR OWN WORK!\n\nseriously it isnt that difficult of a question if you look it up and research it on your own
21761
I started to write a simple answer, but there really is not a simple answer.  Try the sites listed below and see if one makes any sense to you:
210762
Doesn't look like any one knows this answer. It doesn't help asking it twice!!
146529
Kindly refer to :\nhttp://www.sciencenews.org/articles/20000429/note12ref.asp\nYou will find your answers.
57584
The 'How Stuff Works' website has a really good explanation.  You can find it at http://home.howstuffworks.com/question420.htm
167320
Im too long in the tooth to answer this.\nSorry I hope you get a good answer.
359102
my kids find stuff like that in the yahooligans games.  go to yahooligans, then click on games, then go to test your knowledge.  hope this helps
448189
Your question requires more knowledge of science and genetics than I have.  However, since you haven't received much help with it I thought I would give you a couple of sites.  Try these - they may be of some help to you.  Sorry I couldn't help more.
48777
Hi, the link below will explain things for you... sorry for the canned response....
590639
You know, MythBusters did an experiment on this once.\n\nhttp://kwc.org/mythbusters/2003/09/mythbusters.html\n\nRead the second bullet point.
513702
This is a question which the scientific community cannot answer. The answere lies with those who are close to thier religions.