Normally this would be an inane question. It’s kicking on because the computer is getting hot. The question is, why?
I use a Podium Pad and so the underside of the laptop is never resting on a table. I haven’t been watching a DVD or playing a CD-ROM or doing anything else that would make the machine get hot.
It’s an older Titanium PowerBook G4. The monitor is only at 50% brightness. What would cause the cooling fan to keep kicking on? Is it indicative of component failure?
Open up Activity Monitor (Applications --> Utilities) and see if the CPU usage is spiking. I notice that my MacBook will start running quite hot when I’m using Flash (for YouTube, for example). You might be surfing to some CPU-intensive sites.