2. Choose two different numbers from $1,2,3,4,5,6,7,8,9$, then the probability that the sum of the two numbers is even is $\qquad$
Notice that if two numbers are drawn with the same parity, then \( p=\frac{C_{5}^{2}+C_{4}^{2}}{C_{9}^{2}}=\frac{16}{36}=\frac{4}{9} \).