Al and Bob are playing Rock Paper Scissors. Al plays rock. What is the probability that Al wins, given that Bob plays randomly and has an equal probability of playing rock, paper, and scissors?
1. Identify the possible outcomes for Bob's play. Since Bob plays randomly and has an equal probability of playing rock, paper, and scissors, the probability for each outcome is:
   \[
   P(\text{rock}) = P(\text{paper}) = P(\text{scissors}) = \frac{1}{3}
   \]

2. Determine the condition under which Al wins. According to the rules of Rock Paper Scissors:
   - Rock beats Scissors
   - Paper beats Rock
   - Scissors beat Paper

   Since Al plays rock, Al will win if and only if Bob plays scissors.

3. Calculate the probability that Bob plays scissors. Given that each of the three outcomes (rock, paper, scissors) is equally likely, the probability that Bob plays scissors is:
   \[
   P(\text{scissors}) = \frac{1}{3}
   \]

4. Therefore, the probability that Al wins is the same as the probability that Bob plays scissors, which is:
   \[
   P(\text{Al wins}) = \frac{1}{3}
   \]

The final answer is \(\boxed{\frac{1}{3}}\).