Aaron was instructed to burn incense on the altar each morning and at twilight every day as a regular offering to the Eternal. God provided the specific formula for making the incense, stipulating that no other incense was to be burned on the altar. The fire used to burn the incense was always to be taken from the altar of burnt offerings outside the sanctuary. Never was the altar of incense to be used for a burnt offering, a grain offering, or a drink offering. Once a year, on the Day of Atonement, the high priest was to put blood on the horns of the altar of incense to cleanse it, and it was called most holy to the Eternal. Nadab and Abihu disobeyed by offering unauthorized fire before the Eternal, which cost them their lives. More important than burning the proper incense at the proper time with the proper fire and implements was having a proper heart before God. The altar of incense in the wilderness Tabernacle reminded the Israelites that prayer must play a central role in the life of God's people. God gave Moses detailed instructions for its construction, made of acacia wood overlaid with pure gold, standing in the Holy Place. Priests brought burning coals from the brazen altar in the courtyard, carrying them in a censer, and refilled the special mixture of incense morning and evening, so a sweet-smelling smoke issued day and night. Although in the Holy Place, the fragrant smoke rose above the veil into the Holy of Holies and sometimes carried outside into the court, reminding the people offering sacrifices that their prayers were constantly ascending to God.