A three-way bulb is actually two light bulbs in one. For example, a 150-watt, 3-way bulb usually has both a 50 Watt filament and a 100 Watt filament. To get the maximum brightness (150W), both filaments are powered at the same time. If you look at the base of a 3-way bulb, you'll see 3 contacts instead of 2: 100W input, 50W input, and the electrical return line (common to both filaments). That's why if you stick a 3-way bulb into a normal 1-way light socket, it looks dim: only one of the 2 filaments is lit. Three-way bulbs use the same amount of electricity as the equivalent watt single bulb, so the 150W setting takes as much electricity as a normal 150W bulb. The advantage of a 3-way bulb is that you only need to turn on the amount of light that you need, so you can save electricity. The disadvantage is that if you burn out one of the two filaments, then you only have 1 choice left.

To start with how it works, a three-way light bulb has not one (like a standard light bulb) but two filaments inside. As you probably know, filaments can be made to be more or less bright. The filaments of the three-way light bulb have a different brightness; we could say "dim" and "medium". So there you have two levels of light output (each filament on its own), and a third one is added by having both filaments "on" at the same time, where "dim" plus "medium" gives"bright".

The other part of the question was about "using more electricity". The exact way to say this, as you will likely learn in your class, is to talk about using power, and electric power is measured in watts and sometimes just called"wattage". The brighter a light bulb, the larger the wattage - you can check this with some spare light bulbs at home, which normally have the wattage printed on them. So yes, a brighter light bulb uses more power, or "uses more electricity" as one may say colloquially. Also, while you are at it, compare an old-style, filament-based (or incandescent) lightbulb with one of the newer,energy-saving lightbulbs that are essentially compact fluorescent lights. It used to be that you could just tell how bright a lightbulb was from the"wattage" number. The new bulbs, however, use a lot less power for the same light output, so you will often see an "equivalent to xxxW incandescent bulb"notice on the packaging. Compact fluorescent lights now even come as three-way lightbulbs, to come back to your question, where they use the same principle I have described for filament-based bulbs.