Nadab and Abihu offered up incense to the Lord by way of a “strange fire.” The Lord does not allow anything “strange” to approach Him. He had only one authorized ignition source in the entire tabernacle system: the fire on the altar of burnt offering. This fire had been lit by the Lord Himself on the inaugural day of the tabernacle. “Fire from heaven” is the biblical term. We call it a lightning.
Aaron had four sons: Nadab, Abihu, Eleazar, and Ithamar. Nadab and Abihu were the older two and, like all good sons, they were training to go into the family business. They must have shown promise, because the Lord invited them to come along with the elders of Israel for the feast on Mount Sinai. One can imagine how getting an invitation to have supper with God Almighty might cause one to become a little prideful.