Why Google funds open source development?

[Quim Gil <qgil interactors coop>]

Sorry for the wide topic, but considering thet GNOME has got a Google's
Summer of Code fund I think this is an appropriate list to ask.

Why Google is funding open source development? Why Google is funding
GNOME? Just because we are so cool and smart? Just to deduce some taxes?

Has Google given a general explanation? Has Google explained to the
GNOME Foundation why is giving us money?

All the (very few) infos I've read about the Summer of Code talk about
free projects, young developers and money... but they don't mention why
Google is funding them.

Quim