Depends on how you heard the joke :-)
But here's how I heard it:
Every morning the Trids got up, ate breakfast, and marched over the bridge to Tridville to work. One morning, a troll moved in under the bridge. When the Trids tried to cross the bridge, the troll climbed up and kicked the Trids all the way back to their homes. The Trids decided to take the day off in hopes that the troll would go away, but the next morning the troll once again climbed up onto the bridge and kicked them back to their homes. In desperation, the Trids decided to ask the Rabbi for help. So the next morning the Rabbi walked across the bridge several times but never saw the troll. He went home believing the troll had indeed moved on. When the Trids tried to cross the bridge afterward, the troll climbed up again and kicked the Trids back home. The Rabbi returned to the bridge and called out for the troll. When the troll appeared, the Rabbi asked why he was allowed to cross the bridge but not the Trids. The troll replied, "Silly Rabbi, kicks are for Trids."