First, God gave the Israelites a set of very detailed instructions on selecting and preparing a sacrificial lamb. Then, God told them to mark their doorposts with some of the lamb’s blood. By that sign, God would know to pass over their houses when he went around and killed all the firstborn. In addition, God told the Israelites to eat the lamb roasted along with unleavened bread and bitter herbs. Collectively, these observances were (and are) the Passover. Moses said, “You shall observe this rite as a personal ordinance for you and your children. When you come to the land that the LORD will give you, as he has promised, you shall keep this observance. And when your children ask you, ‘What do you mean by this observance?’ you shall say, ‘It is the passover sacrifice to the LORD, for he passed over the houses of the Israelites in Egypt, when he struck down the Egyptians but spared our houses.’ And the people bowed down and worshiped.” (NRSV, Exodus 12:24–27) After God killed the firstborn, the Egyptians cried out and urged the Israelites to leave. So they did.