Chemical Concepts Demonstrated: exothermicity, liquids that expand when forming gases, effects of sparks on combustible gases and volatile liquids
The corks at the top of the bottles fly into the air when initially "sparked." When the bottles are corked and sparked again, the rockets no longer launch. When the bottles are tipped over and placed upright again, the corks launch again.
Explanation (including important chemical equations):
This reaction is initiated with a spark:
2 CH3OH(l) + 3 O2(g) -> 2 CO2(g) + 4 H2O(g)
At any given temperature, gases occupy a volume that is about 800 times larger than the corresponding liquid (even moreso in this case because of the additional moles produced). The reaction produces enough pressure very quickly to cause the corks to shoot off like rockets.
When the second attempt is made, there is too much CO2 and not enough O2 in the bottles to initiate the reaction.
When the bottles are tipped over, CO2 pours out and O2 comes back in, allowing the reaction to run again.