Here are some numbers: 0, 1, 2, 2.5, 3 and 4…

(a) Molecularity. The possible values are $1$, $2$ and $3$. Particles are counted, so the number is whole, and at least one particle must take part. A meeting of four particles at once is so unlikely that no such step is known.

(b) Order. All six numbers are possible. The order is the sum of the powers in an experimental rate law. A power may be zero, a whole number or a fraction. Examples: order $0$ for a reaction on a surface that is already full, order $2.5$ for a reaction with a rate law such as $k[\text{A}]^{1/2}[\text{B}]^2$.