Today my son asked me to define the word 'accident' to him. I did and followed up with teaching him what 'on purpose' meant as well.
I'm here to tell you that everything he does today is followed up with these two words: on purpose.
"Look Mom, I'm riding this bike on purpose."
"Look, Mom, I'm eating on purpose."
"Can you seeeeeee that I'm picking up this crayon on puuuurpooose?!?"