On a family outing, mum and dad were riding on the bike and son was traveling in the sidecar.

"Dad, what's that?" said the son.

"You tell 'im." says mum

"Dad, do you know how many yellow cars we have passed?"

"You tell 'im." says mum

"Dad, how long before we get there?"

"You tell 'im." says mum

Dad says to the son, "If you open your mouth once more on this journey, I'll scalp your hide."

When they eventually arrive at their destination, dad looks round and screams at the son, "Where's your mother?"

"She fell off about 5 miles back"

"Oh well", he says, "Could have been worse. I thought I'd gone deaf."

So what's the joke?