Graphite is a soft, dark gray form of pure carbon that we use most commonly in pencils. When you write with a pencil, you're actually leaving a trail of graphite on the paper, not lead (despite people calling it “pencil lead”). The graphite is mixed with clay and baked into the thin rods inside wooden or mechanical pencils.

What makes graphite special is how its carbon atoms are arranged in slippery layers that slide past each other easily. This is why it writes so smoothly and why it's also used as a lubricant in locks and machines. The more clay mixed with the graphite, the harder and lighter the pencil mark; less clay makes darker, softer marks. That's what those letters and numbers on pencils mean: a 2B pencil has more graphite and writes darker than an HB pencil.

Graphite is also an excellent conductor of electricity and can withstand extreme heat, so it's used in batteries, electronics, and even spacecraft. It's the same element as diamond, just arranged differently: diamond's atoms form a super-hard crystal structure, while graphite's atoms form soft, slidable sheets.