Answer is: a hill over which a wagon is pushed.
For all chemical
reaction some energy is required and that energy is called activation
energy (energy
that needs to be absorbed for a chemical reaction to start).
There are two types of reaction: endothermic
reaction (chemical reaction that absorbs more energy than it releases)
and exothermic reaction (chemical reaction that releases more energy than
it absorbs).
Reactions
occur faster with a catalyst because they require less activation energy.