What makes water boil?

Basically how boiling takes place?

Also like to know...
What makes boiling point alter at various altitudes?
Why bubbles rise through boiling water?

Heat - We know that Heat is a form of energy that is transferred by difference in temperature of regions. Here, it's just the amount of energy required by the $H_2O$ molecules in the liquid state to get converted to gaseous state. As we provide heat, it gets absorbed by water to get converted into water vapors.

Convection - It's the transfer of heat through water (proper term is fluid) caused by molecular motion. You see, Convection can be brought to focus by taking normal experiments into account. Particles (even Chalk) would rise and fall in somewhat rotatory motion due to this flow of heat. The water vapor formed at the bottom of the liquid (water) flows to the top in the form of united water vapor bubbles. 'Cause it's a gas and its so hot down there..! (This explains bubble formation when water is boiled.)

Now, why boiling point differ with altitude:
The atmospheric vapor pressure is 1 atm only for us (I mean, at sea level). For higher elevations, the pressure is too low out there in atmosphere. It should also be noticed that the volume change is too high when a liquid changes to gas.

By Ideal gas equation, For 'n' moles of gas $\implies$ $P=\frac{nRT}{V}$ which shows that pressure is inversely proportional to volume. So, we could relate this expansion with pressure. Expanding against low pressure takes somewhat lower energy than expanding against high pressure. And hence the boiling point would be low at higher altitudes. Wiki says it somewhat clearly with a formula.

Using this water boiling point calculator, Boiling point at sea level (1 atm or 29.92 inches of Hg) is approximately 212 °F whereas at a height of about 5000 ft. (24.9 inches of Hg), it is 203 °F.