"Crazy Handful of Nothin"
Season 1, Episode 6

Krazy-8 is dead, and Tuco Salamanca has replaced him. Jesse heads to Tuco's headquarters and gets knocked around after Tuco refuses to pay him for the Blue Sky. Meanwhile, Walt is back at home, watching his hair fall out in clumps from the top of his head. Walt's losing a grip on his hair, and losing a grip on his business. So, when Walt finds out what happens to Jesse, he adapts: he shaves his head, grabs some meth, and heads to Tuco's himself. 

Once there, Walt introduces himself to Tuco as "Heisenberg." Tuco laughs Walt Heisenberg off for bringing him more Meth after kicking Jesse's ass, but what he doesn't know is that what Walt brought isn't meth: it's fulminated mercury, which looks like a batch of crystal. After epically announcing, "This isn't meth," Heisenberg takes a bit and throws it violently, causing a chemical reaction that leads to an explosion. Intimidated, Tuco gives in to Heisenberg's requests. When Tuco asks him what just happened, Heisenberg simply replies, "A little tweak of chemistry."