Once a year Aaron must put the blood of a sin offering on the horns of the altar. He must do this to make the altar pure. He must do this on the day Israel’s sin is paid for. Do this for all time to come. The altar is a very holy place to me.”
Nadab and Abihu were two of Aaron’s sons. They got their shallow cups for burning incense. They put fire in them. They added incense to it. They made an offering to the Lord by using fire that wasn’t allowed. They did it against his command.