Gene said that a dirty torch tip can cause popping, as will trying to weld with a tip that is too large. Next, we tried our hands at lap welds. This involved putting one piece of practice stock over another offset somewhat, and welding the top piece to the bottom along the edge of the top piece. After welding quite a few lap welds we progressed to T-joint welds, in which one piece stands on it side on another piece and the weld is made along the joint. These different types of welds are progressively more difficult, because of the difference in heat dissipation from a weld site in the middle of a piece of metal as opposed to the dissipation from a weld on the edge.
Heat can only dissipate in one direction from an edge, making it much easier to burn through.
It took me a lot of practice to keep from burning through, yet get good penetration in the weld. Kearns demonstrates basic butt welding techniques. Filler rods come in varying thicknesses about 30 inches long and are melted in the flame along with the base metal. Do this correctly, and the weld looks like a series of dimes laid overlapping in a line. Do it poorly, as I did quite a few times, and the rod sticks in the weld, or creates uneven globs and craters in the weld.