Water generally flows from rivers to the sea, everyone knows that...
OK, maybe not everyone, but you can trust us here at Newsround when we say it does.
But not always, there's a natural phenomenon called a tidal bore which sends water in the other direction.
It happens at the point the river meets the sea when the waves of the sea are so strong that they push water up-stream against the river's current.
It's most obvious at particularly high tides when the Moon and Earth are lined up.
Huge waves were seen on the Qiantang River this week and thousands of people went there to see the massive tidal bore.
We can promise it's not boring! Take a look.