A shell explodes by means of a process called the explosive train. This process consists of a series of explosions that detonate the shell after the projectile has been fired. The explosive train begins with the explosion of the trigger. The trigger may explode the instant the shell hits the target, or it may detonate a few seconds earlier or later. Some armor-piercing shells have a delayed fuse, which enables the projectile to penetrate before explosing. Most fuses operate mechanically or electronically. Mechanical fuses are activated by the movement of the shell during launch from the weapon, and the rotation of the shell as it travels through the air Electronic proximity fuses are activated by devices inside the shell that use a radar waves to determine when the projectile is near the target. In most shells, the fuse ignites the primer and thus sets off the first charge in the explosive train. Each successive charge in the process is more powerful that the previous one. The amount of force generated by the explosion of the charges increases until enough power has been created to detonate the main charge.