Go stand in front of a mirror right now. If you make an angry face at the mirror, will the mirror smile back at you? No! It will give you back the angry face. If you're wearing a Spider-Man mask, the mirror will show Spider-Man.
The Bible says our life is just like that mirror. If inside our heart we're full of anger or lies, our life will end up "ugly" (fights, punishments). But if our heart is full of joy and of Jesus, our life will be beautiful and happy, just like a smile in the mirror. What comes out is what's inside!