"Lifeguard's Secret" joke

This young man went to the beach in search of girls, and after strolling around
for some time, he wasn't even getting a glance from any of them. He began to
feel kind of discouraged, and he noticed that they were flirting with the
lifeguard.
He walked up to the lifeguard and asked him how he was able to attract so many
girls. The lifeguard said that the secret was to put a potato inside of his
trunks.
So the next morning the young man put a potato in his trunks and went back to
the beach. This time the girls were giving him disgusted looks and were turning
away from him.
He went back to the lifeguard and asked why it didn't work. The lifeguard said,
"Try it again tomorrow and this time, put the potato in front".