Final answer:
Particles must collide with an energy equal to or greater than the activation energy needed for a reaction to occur.
Step-by-step explanation:
In order for a reaction to occur, particles must collide with an energy equal to or greater than the activation energy needed for the reaction. This is because the activation energy is the minimum energy that colliding particles must have in order to undergo a reaction. If the colliding particles have less than the activation energy, the particles will rebound and no reaction will occur.