To play a joke on [this question](https://superuser.com/questions/295851/can-i-use-one-laptop-to-charge-another-laptop-by-connecting-the-two-together-with), we came up with the following image on the [Super User Chat](http://chat.stackexchange.com/transcript/message/1133217#1133217).

![](https://i.stack.imgur.com/xAAaH.png)

Of course this will not work, but now my question is:

**What happens if you charge a laptop using it's USB power?**

Assume there is a fully charged battery in the laptop to begin with. ;)

I see three possibilities:

1. A short circuit happens, the cable might map - on + and vice versa.
2. Nothing happens, like you were to connect the + and - of a battery to itself.
3. Power goes around in a circle and you lose faster than usual due to heat.

But I can't see which one of those would really happen...