Fans run after Macbook Pro lid is shut

The fans in my MBP will continue to run for a minute or two after I close the lid. This is frustrating because I often want to close it and immediately put it away in its soft zipper case but I don't want to put it in the case while it's still spitting out hot air. Any ideas why the fans don't turn off immediately (or within like 15 seconds)? This started happening after I upgraded to 10.9.

Solution 1:

This typically happens when the CPU has been working and the system needs to push more hot air out. I think the firmware within the machine manages this but I'm not quite sure. My air does it and I just put it away anyway. I've never experienced anything bad with it.