To find the probability that both spark plugs you buy are defective, you can use the given probability that a new spark plug is defective, which is 0.06.
Assuming that the two spark plugs are independent, the probability that both spark plugs are defective is calculated by multiplying the probability of one spark plug being defective by itself.
So, the calculation is as follows:
\[ P(\text{both defective}) = P(\text{first defective}) \times P(\text{second defective}) = 0.06 \times 0.06 = 0.0036 \]
Therefore, the probability that both spark plugs you buy are defective is \( \boxed{0.0036} \).