While reading the introduction to Feynman's lectures, it's mentioned how a glass of water cools down through evaporation, when some molecules get a bit extra energy and break free. If it's not a closed system, energy will be gradually taken away from the cup, hence blowing at the soup helps move those molecules away so that they don't reenter the surface.
But I thought that all bodies also radiate heat? Does a cup of water also emit low frequency radiation, or is my understanding incorrect?
Yes, all matter above absolute zero emits radiation. To quote wiki:
When the temperature of a body is greater than absolute zero, inter-atomic collisions cause the kinetic energy of the atoms or molecules to change. This results in charge-acceleration and/or dipole oscillation which produces electromagnetic radiation, and the wide spectrum of radiation reflects the wide spectrum of energies and accelerations that occur even at a single temperature.
This continuous release of energy would eventually cool the source to a lower and lower temperature except your glass of water is in contact with a heat reservoir (the room) which compensates for the energy loss.
As @lemon explained, every body emits electromagnetic radiation if its temperature is above absolute zero
The peak wavelength of the radiation emitted by a blackbody is given by Wien's law: $$\lambda=2900/T$$ where T is the temperature in K, and the wavelength ($\lambda$) is in $\mu m$.
For a temperature of 300K (27C), approximately room temperature, the peak wavelength is $9.7\mu m$.
More details can be found at this site. The site has a calculator that shows the peak wavelength, given the temperature, as well as a graph that shows the spectrum at the selected temperature.
