Most laptops dissipate heat through the bottom.
If you put a running laptop on a bed, it will probably overheat because no air will circulate under the bottom. However, results will vary. Your laptop will probably run cooler outdoors in 100 degree heat in a gentle breeze than in your breezeless air conditioned office. Posted on Apple's website, reprinted with author's permission. I just bought a device called Road Tools' CoolPad.