Undertow (water waves)

Imagine you are in a big swimming pool and you start splashing with your hands, making big waves. When the waves reach the edge of the pool, they crash and then they start going back towards the middle of the pool. This back and forth movement of the water is called an undertow.

When you are at the beach, the same thing happens with the waves from the ocean. The waves come crashing onto the shore, but then the water starts flowing back towards the ocean. This flow of water can be very strong and pull you away from the shore if you are not careful.

It's important to be aware of undertows when swimming in the ocean because they can be dangerous. If you feel the water pulling you out towards the ocean, it's best to swim parallel to the shore until you are out of the undertow, and then swim back towards the shore.

So just like in the swimming pool, when waves crash onto the shore, they create an undertow, which can be strong and pull you away from the shore.