EE was climbing down the steps of the water truck. As he placed his left foot on the ground, he felt his knee pop. Supervisor saw EE limping. EE said he would be fine continuing with his job. Approx 30 min later the Supervisor observed limp was more noticeable so he took EE to the ER. Taken off work for 2 days so he could follow up with his DR. 9/13/15 knee popped back in place.