I left my iBook unattended for about 15 minutes, with just Safari open with the Macrumors homepage showing. I had previously been browsing beforehand however. So after being online, leaving the window open, and coming back later... I heard the fan on, and the temperature I read was in the 160's. This has never happenned in the 1.5 years I've owned this. What could be the cause of this?