I've never noticed this before and I'm assuming swimming is a hidden skill. My dad sim swims laps a lot to stay in shape and he loves the outdoors. He and his son were both swimming and I thought I'd have them have some fun and used the Splash interaction. I was going to do it again (Dad clicking on son) and noticed an interaction called "Coach Swimming." Curious, I tried it. It functions like any other mentoring. The son is now swimming laps with Dad 'coaching'. Dad has the Great Teacher (from Mentoring) buff but the son has no buff from the mentoring but seems to be leveling his motor skill faster. Just thought I'd share as I hadn't seen it before.
