Can I modify document.cookie in the console of Chrome Developer Tools?

My current cookie string was like:

coldcookie=

It seems it just doesn't work if I run this code below:

document.cookie = document.cookie + ; newcookie=something

The document.cookie wouldn't change at all.

Update: I found that if I run:

document.cookie = newcookie

It actually add a newcookie in the cookie string like:

oldcookie=; newcookie

Shouldn't that clear the current cookie string?

It does the same thing in IE. So I think there must be some rule there. Any ideas?