I liked very much the idea of giving a variation to the desert
problem. Here is a couple of suggestions (in both of them I had the
round-trip version of the problem in mind):

1. Suppose that the tank of the truck is not well sealed and some of
   the gas leaks as you travel. In particular, say that

	you lose  x  gallons of gas every time you travel 
        the distance that  x  gallons allow you to travel.

   This is essentially your idea about the gas evaporating, but it's
   slightly better as you don't need to explain why the gas does not
   evaporate when deposited at the stations.

   It is a very easy variation, eventually giving you 1/4 of the
   harmonic number (I think). But a student that hasn't understood
   the setting won't be able to solve it. It makes a good first part
   to a problem that will have more parts, with more difficult
   variations of the desert problem or as easy variations of other
   problems. 


2. Our strategy was: at every stage, use  n+1  tanks of gas from the
   current station to store  n  tanks of gas at the next station,
   making  n+1  small trips between the two stations. 

   What if we follow the following strategy: at every stage, use
   2^{n+1}  tanks from the current station to store  2^n  tanks of gas
   at the next station, making  2^{n+1}  small trips between the two
   stations. 

   If you do this, then:
   
   the _distance between successive stations_ becomes constant:  1/4
   (as opposed to increasing from 1/n up to 1/2 in the original
   problem), 

   but 

   the _number of small trips between successive stations_ is
   decreasing exponentially from a very large starting number 2^{n+1}
   down to  1  (as opposed to decreasing polynomially from a small
   number  n  down to  1). 

   Hence, you can start asking questions: 

   * Suppose building a station is expensive (or this is war, and the
   enemy's aircrafts can spot your stations and blow them up, so the
   fewer you make the better) and we want to minimize the # of
   stations that we need, which strategy is better?  A: For the same
   desert, the original strategy needs exponentially more stations
   that the new one.    

   * How does the total # of small trips that you make change between
   the two strategies?  



