John know the age differences between he and mommy. He also like to wash hairs at the same day with Erica.
One day, we went to cut hairs and thus didn't need to wash hairs at home again.
John was angry as he needed to wash hairs that day.
I recalled some articles used to say that laugh is good for health.
Dad: John, if you get angry once, you will become one year older. The age differences between you and mommy will become shorter.
John became more angry after hearing my words.
I kept adding one more year for him when I saw his angry face.
When I added to 10 years-old. I told him,
Dad: John, you need to start laughing asap, each time you laugh, your will become one year younger. If you don't start to laugh, you will become older than mommy very soon.
After a while, I saw that John's mouth is trying to change to laugh though not in a natually way. I reduced one year for him. I reduced one more year for him whenever I saw his mouth look like laughing.
When he heard that his ages became less then 5, He bursted into laughter.
I don't know why he so care about the age differences between he and his mommy. However, I try the same way twice. It always works though I don't know how long the same way can be worked.