The following example is given in my textbook:

Question

If a chemical equation is:
NO2 (g) + CO (g) -> NO (g) + CO2 (g)
The rate law is:
Rate = k[NO2]^2

Usually you put in the reactants in their own sets of brackets, so why isn't the reactant CO involved in the rate law?

I get it now - the order of CO is 0 and anything to the 0 power is 1.