Up pulled a car and out came Chris Kuhn, a classmate Sheen disliked for bragging about his fighter pilot father.

"He'd bring in his father's head gear or the whole pilot outfit," Sheen recounted Wednesday at Chaminade-Julienne High School. "He made us all feel so insignificant and jealous."

But there on the sidewalk that day, the unnoticed Sheen watched as Kuhn's father put the car in park, got out, walked around to the sidewalk and kissed his son on the lips.

"What a public display of parental love," Sheen said.

"I was struck by it. And to this day if any one of my children or grandchildren comes through the door I kiss them right smack on the lips."

The lesson Wednesday for an auditorium packed with an estimated 500 students.

"You never know when you are doing something that is affecting someone," Sheen said.

That was the actor and Dayton native's call to arms for social activism. After a ceremony honoring Sister Dorothy Stang, a nun killed last year in Brazil, Sheen and Sister Rebecca Spires, who worked with Stang in Brazil, and Emily Goldman, a human rights lobbyist, spoke to the students.

Answering a question from senior Krista Seaman, Spires said she first realized she could make a difference through activism when family and friends disapproved of her first black friend.

"Right then and there I decided I was going to be on the other side of this whole society," she said.

Another student asked why work for social change when it seems hopeless that one person could make a difference?

"This is really the fundamental question for all of us," Sheen said. "How do you make a difference and what difference does it make? This has to be highly personal. The only thing you can change is you."

Sheen, who has been arrested dozens of times for protesting on behalf of social issues, urged the students to look inward for what they might do to help their communities or the world.

"You have to look inside and say, 'I cannot not do this and be myself,' " he said. "I don't even think about trying to change other people's minds. I do it for myself."