This was the moral value that one devoted teacher set out to instil in her pupils one day in class.
Using nothing but apples, the Relax Kids Tamworth teacher, from Staffordshire in the UK, perfectly illustrated the physical and emotional effects of bullying.
"I introduced the children to two apples," she wrote on Facebook, adding that she had repeatedly dropped one of the apples on the floor without the children's knowledge.
"I picked up the apple I'd dropped on the floor and started to tell the children how I disliked this apple, that I thought it was disgusting, it was a horrible colour and the stem was just too short. I told them that because I didn't like it, I didn't want them to like it either, so they should call it names too.
"Some children looked at me like I was insane, but we passed the apple around the circle calling it names, 'you're a smelly apple', 'I don't even know why you exist', 'you've probably got worms inside you' etc."
They really pulled the "poor apple apart" till she "actually started to feel sorry for the little guy."
They then passed the other apple around, saying kind words to it.
"'You're a lovely apple', 'Your skin is beautiful', 'What a beautiful colour you are' etc."
Afterwards, the teacher held up both apples, and the apples still looked the same.
"I then cut the apples open. The apple we'd been kind to was clear, fresh and juicy inside," revealed the teacher.
On the other hand, "the apple we'd said unkind words to was bruised and all mushy inside".
"I think there was a lightbulb moment for the children immediately. They really got it, what we saw inside that apple, the bruises, the mush and the broken bits is what is happening inside every one of us when someone mistreats us with their words or actions."
The teacher then went on to explain that when people are bullied, especially children, "they feel horrible inside and sometimes don't show or tell others how they are feeling".
"If we hadn't have cut that apple open, we would never have known how much pain we had caused it."
She also shared her own experience of "suffering someone's unkind words" the previous week.
"On the outside I looked OK, I was still smiling. But, on the inside someone had caused me a lot of pain with their words and I was hurting.
"Unlike an apple, we have the ability to stop this from happening. We can teach children that it's not OK to say unkind things to each other and discuss how it makes others feel.
"We can teach our children to stand up for each other and to stop any form of bullying, just as one little girl did today when she refused to say unkind words to the apple."
The post has gone viral, garnering more than 145 000 shares and 131 000 reactions.
"More and more hurt and damage happens inside if nobody does anything to stop the bullying. Let's create a generation of kind, caring children.
"The tongue has no bones, but is strong enough to break a heart. So be careful with your words."