You know when you pick a beautiful fruit, all red and shiny, but when you bite it, yuck, it's rotten inside? On the outside it looked perfect, but what really mattered was hidden on the inside.
It's similar with people: someone can seem nice on the outside, all smiles, but be keeping bad things in their heart, like anger, lies, or the urge to do mean things. Other people only see the outside, but God is different: He sees our heart, what we truly feel and think.
That's why it's not enough to just pretend to be good; what matters is having a truly good heart. And the most beautiful thing is that God is happy when our heart is good and honest, even when nobody's watching. And if our heart needs a little help, all we have to do is ask, because God loves making our heart more beautiful on the inside.
It's not enough to just seem good; God looks at the heart and loves those who are truly good.