Just when the Israelites thought it was safe to go back to the rules for holy water, the initiation of Aaron and his sons into the priesthood takes an unfortunate turn.
Nadab and Abihu, Aaron's two oldest sons, offer "strange fire" that isn't in keeping with God's instructions.
God sends out fire to burn them up and they die.
If the story of Nadab and Abihu were a spec script, a studio would probably reject it for being too on the nose. After all, it's pretty convenient that after nine chapters emphasizing the importance of following instructions, two guys get fried for not following the rules.
Leviticus expert Jacob Milgrom notes that one of the main points in this story is the importance of ritual purity, which sets up the book's next few chapters.
In Hebrew, Nadab and Abihu take "coals from an outside place"—that is, not from the holy altar but from a not-holy-fied area such as their uncle's beat up George Foreman grill.
Afterward, God tells Aaron and his remaining sons that if they don't want to get killed, priests must never drink wine or other fermented drinks when going into the tent of meeting. Despite all the frankincense, God is earning a reputation as a serious buzzkill.
God also orders them not to mourn, but to carry on with the remaining sacrifices as required. Worst. Boss. Ever.
The chapter ends with an argument between Moses and Aaron. It turns out, after their brothers got zapped, Aaron's remaining sons don't eat the goat meat from the sin offering.
Moses gets ticked off over the fact that they don't appear to be respecting his orders, especially since the whole point is for the priests to eat this meat as part of the sin offering's process of atonement.
Aaron gets the better of him, however, by noting that after what happened, God wouldn't be happy with him eating the sacrifice.
Ritual purity also seems to be why Aaron is able to layeth the smacketh down on Moses. The corpses of Nadab and Abihu made the Tabernacle unclean.
Eating sacrificed meat in a polluted Tabernacle without additional cleansing could have sparked more fatal cleansing fire. As the old saying goes back in Tarshish, fool me once, shame on me—or something like that.
The subtext, though, is the superior authority of priests, or at least those that God hasn't chosen to burn. Not only does God speak directly to Aaron after an impressive inauguration ceremony, but the story shows Aaron to be smarter than Moses when it comes to interpreting God's law.