River of rockets

I was asked this morning, by an 8 year old, how the moss forms on the river and why it doesn't flow away with the current.

It was one of those questions where you puff your chest out and confidently ask said questionnaire if he saw that rocket pass through the sky above us.

I haven't got a clue about the answer but thankfully he forgot he ever asked the question as he starred at the sky for 5 minutes...

Phew..