Jump

When we go to the beach, Jack loves to jump over the waves as they crash onto the shore.
A bit of fun.
Then you start doing it yourself, cause he wants you to. Then you start to enjoy yourself, you see how high you can jump, jump with a side kick, jump with a 180 degree turn, add some sound effects to the jump...
Then Jack says he wants to build a sandcastle.
You get away with jumping over small waves whilst doing weird hand movements, karate kicks and making a strange sound that sounds like a cross between a cat and a chainsaw, as long as a four year old is diong it with you.
By yourself, you just look stupid.
"Sandcastles? Oh, OK."
"I wonder how big we could make the trench around it? Maybe so a real boat could float on it!"
"Dad!!"