The waves crash on the beach at different levels throughout the day but in general don’t go beyond their usual routine boundaries. This constant predictability is something that many people who have jobs relating to the sea depend on. Tourists as well depend on these boundaries and predictable routines for the sake of their holidays.
Kids need boundaries. They work well with them and are confused without them. These are things that have to be learned though, and consistency is important.
Part of the reason is that they are learning what is and isn’t concrete. At eighteen months, my son would drop things from many different heights to see if they would still fall. Kids at this age will often jump and hurt themselves unintentionally and have to be more carefully monitored. They are simply trying to figure out something we as adults take for granted: Gravity.
Talk with your child often during this age. They are learning communication skills and will start to understand. But be firm in your explanations and let them know that certain laws (like gravity) will not change. They don’t know this and often will try and test it by dropping, throwing etc. to see if the object still falls. Sometimes they will do this over and over and with many objects.
You can get a ball or other toy if you like, and show them the same results over and over. They will still try it on household items such as glass vases etc, so be sure that they know those are off limits and can hurt them.
Kids are super curious about how the world works. They are natural little scientists. They will experiment with their hypothesis over and over until they are convinced it is firm in all circumstances. These are good observation skills that they need to develop for the future.
But don’t give them free reign to experiment on your nice China. Firm boundaries, even if they have to repeated over and over pay off. Give them freedom instead to drop something else instead.