Whether you're signing up to comment, to write articles, to blog -- people like to know a bit about you. Your bio is a place where you can tell people. You can talk about your work , your passions, your family, your activism, your hobbies, your favorites, your age, marital status, beliefs, books you've written, accomplishments in your life, favorite quotations. Have fun with it. (Your bio IS made public. And will appear at the end of articles you submit. It is website policy to NOT permit inclusion of your email in your bio -- to prevent spam and misuse of the website. We DO want your email, but only in the email field just above.)
