Answer the following question given this paragraph: 

The behavior of the atoms, molecules, or ions that comprise the reactants is responsible for the rates of a given chemical reaction. Collision theory is a set of principles that states that the reacting particles can form products when they collide with one another provided those collisions have enough kinetic energy and the correct orientation. Particles that lack the necessary kinetic energy may collide, but the particles will simply bounce off one another unchanged. The figure below illustrates the difference. In the first collision, the particles bounce off one another and no rearrangement of atoms has occurred. The second collision occurs with greater kinetic energy, and so the bond between the two red atoms breaks. One red atom bonds with the other molecule as one product, while the single red atom is the other product. The first collision is called an ineffective collision , while the second collision is called an effective collision .


Q: According to collision theory, particles that lack what kind of energy may collide, but the particles will simply bounce off one another unchanged?


A:
kinetic energy