Joshua 4:5-7

and Joshua said to them, “Cross over again to the ark of the LORD your God into the midst of the Jordan, and each of you take up a stone on his shoulder, according to the number of the tribes of the sons of Israel, so that this may be a sign among you; when your children ask later, ‘What do these stones mean to you?’ then you shall say to them that the waters of the Jordan were cut off before the ark of the covenant of the LORD; when it crossed the Jordan, the waters of the Jordan were cut off. So these stones shall become a memorial for Israel forever.”