Tsunamis
Tsunamis is a set of huge ocean waves that form when the sea floor shifts fast.
Think of it this way
Drop a heavy book flat onto a full bathtub. The splash pushes a wall of water to each end. A tsunami works the same way — the sea floor shifts and shoves water outward. That water piles up into tall waves as it reaches shallow ground near shore.
Explaining tsunamis by grade level
Think of a wave tank with small buildings on the shore. When you push the bottom of the tank up fast, a big wave rolls toward the buildings. That wave is like a real tsunami. The wave moves through all the water, not just the top.