electrical safety
Electrical safety is the simple, serious habit of not letting electricity hurt you or start a fire. The single most important fact for a beginner: mains electricity from the wall (110 to 240 V) can kill, while typical low-voltage hobby circuits (batteries, USB, 3 to 12 V) are generally safe to touch. The difference is not fussiness; it is the line between a harmless project and a hospital visit.
What injures you in a shock is current through the body, and it takes voltage to drive that current through your skin's resistance. Low voltages such as a 9 V battery cannot push meaningful current through dry skin, so you feel nothing; mains easily can, especially if you are damp or grounded, and as little as tens of milliamps across the heart can be fatal. So the rules: never work on mains-powered equipment live; keep one hand in your pocket near anything high voltage; respect that capacitors can hold a charge after power is off.
Beyond shock, two everyday hazards bite beginners. Heat: resistors, regulators and wires get hot when over their power rating, and a short circuit can make a battery dump enormous current, overheat, and even burst, lithium cells especially. Polarity: plugging a part or a power supply in backwards can instantly destroy it, and electrolytic capacitors can pop. Work at low voltage, double-check polarity, fuse or current-limit your supply, and keep a hand free of the circuit when in doubt.
A 9 V battery across dry skin pushes almost no current and is harmless, but the same battery shorted by a ring or a wire can dump several amps, get scorching hot, and burn you or start a fire.
Low voltage is safe to touch, but a short circuit is still dangerous because of heat.
Treat anything that plugs into the wall as potentially lethal and do not open it live; the comforting low-voltage-is-safe rule applies only to genuinely low-voltage battery and USB circuits, never to mains.