A sample of gas has a particular volume at #0^o# #C#. To what temperature must it be heated, if the pressure is kept constant, to double its volume?
Since the temperature and volume are directly proportional (when the pressure is kept constant), doubling the volume requires doubling the temperature.
This question is tricky, because we are tempted to try to double zero and still get zero. It's one of the reasons the Kelvin temperature scale is important.