It's not really a miracle that happened, the smile changed the situation, it changed your feelings about the day which was making you feel unrelaxed...

When the child was smiling, all your thoughts were channelled towards him, how cute he/she looks, why is he /she just staring at me...
Then you have forgot all that happened some minutes ago in class...

Smile is truly contagious but sometimes we harden our hearts and the smiles might not have effect on us, even words of encouragement, hug etc..

We have of let loose of the bad feeling so as to accommodate other spices of the day..