How fast must I stir the glass of water to get it boiling? I have a glass of water at room temperature and I decided to add more and more kinetic energy by stirring it with a plastic spoon so that it starts boiling... can this be done? if so how fast must I stir? (both manually and mechanically.)
 A: As this scenario examined in what-if xkcd

When you stir tea, you're adding kinetic energy to it, and that energy
  goes somewhere. Since the tea doesn't do anything dramatic like rise
  into the air or emit light, the energy must be turning to heat.
  [...]The reason you don't notice the heat is that you're not adding
  very much of it. It takes a huge amount of energy to heat water; by
  volume, it has a greater heat capacity than any other common
  substance.

In other words, the energy does not go anywhere, it is just a small energy that you are putting into the water that it is negligible.
Edit:

Even if you could churn the spoon hard enough—tens of thousands of
  stirs per second—fluid dynamics would get in the way. At those high
  speeds, the tea would cavitate; a vacuum would form along the path of
  the spoon and stirring would become ineffective. [...] And if you stir hard enough that your tea cavitates, its surface area
  will increase very rapidly, and it will cool to room temperature in
  seconds: No matter how hard you stir your tea, it's not going to get 
  any warmer.

A: I am going to take a very naive approach to answer this question.
Energy of a rotating body, in this case, a cylindrical column of water, can be taken as the energy of a rotating solid cylinder in the roughest approximation.
Rotational kinetic energy of such a cylinder is: $\frac{1}{4}mr^2 \omega^2$.
If this energy is to be used to increase the temperature of the water, then for a mass $m$ of water with specific heat capacity of $c$, undergoing a temperature change of $\Delta T$, 
$$\frac{1}{4}mr^2 \omega^2 =mc\Delta T$$
This leads to 
$$\omega = \sqrt{ \frac{ 4c\Delta T}{r^2}}$$
If we try to use some sensible values in this equation, say, 
 c = 4200 J/kg$^\circ$C, $\Delta T$ = 80 $^\circ$C, and $r$=10cm (0.1 m), then we get a value of 8198 radian/sec for $\omega$, or roughly 1845 revolutions per second, or 110700 revolutions per minute.
