For example the equation;
NO(g) + O3(g) ---> NO2(g) + O2(g)
What is happening in this reaction?
 
  An oxygen atom is transferred from ozone to NO.
How could this happen?
 
  When an NO molecule collides with an O3 molecule an O atom would 
    get transferred. 
How would an NO molecule collide with an O3 molecule?