The RFC or the reason why you can not create CNAME record for the "root record"

[list3 at wwwcrazy.com]

Why can you not create a CNAME record for the "root record".
I'm not sure if I'm using the correct terminology.
But basically I want to know why this command does not work:
update add example.com. 10 IN CNAME www.mysecondexample.com.

I know it must be in an RFC somewhere but I can not see it anywhere.

Thanks,
-Steve