A microplane's teeth aren't stamped into the steel the way a box grater's are. They're etched — a photographic mask defines each tooth's shape, and acid removes the metal around it, so every edge comes off the process already knife-sharp instead of punched blunt. That's the entire difference. Everything else about how the tool behaves follows from it.
Why an etched edge cuts and a stamped edge tears
Ordinary grater holes are punched. A die forces a hole through sheet metal, and the metal around the rim curls up and tears rather than slicing cleanly — that ragged little hook is what grabs a strand of nutmeg or a knuckle. A microplane's teeth are made a different way. A photographic mask protects everything except the tooth shape, an acid bath dissolves the exposed steel away, and what's left is a genuinely sharp edge rather than a torn one. The tool started as a woodworking rasp built the same way, and the company name that came with it is now just the word for any grater made this way, the same drift that happened to trampoline and escalator.
Why it only cuts moving one direction
Each tooth is angled forward, not straight up, the same way a saw's teeth lean one way down the blade. Pull food across that angle and the edge bites in and shears off a curl. Push it back the other way and the tooth's flat side just glides underneath, doing nothing. Sawing back and forth the way you'd work a box grater wastes half of every stroke, and pressing harder on the return pass is more likely to bend a tooth than cut anything. The blade is thin enough that force doesn't help it. Direction does.
Why zest comes off clean and garlic turns to paste
Both come down to the same two things: how far a tooth stands proud of the blade, and how closely the teeth are spaced. Citrus and garlic sit at opposite ends of that. A lemon's colored layer, the flavedo, sits only about a millimeter deep, directly over the bitter white pith. A microplane's teeth are shallow enough to shave off exactly that layer and no more, so one firm, even pass lifts the zest and leaves the pith in place. A second pass over the same spot is what finally cuts into it.
Garlic gets the opposite geometry. The teeth are packed close enough, and enough of them meet the clove in a single pass, that they don't leave shreds the way a box grater does. They rupture nearly every cell they touch, so what lands in the bowl is closer to a paste than a mince — more of the clove's juice worked in, not left behind on the board.
Why grated parmesan fills the measuring cup
Parmesan comes off a microplane as thin, curled threads with air trapped between them, not the denser shreds a box grater produces. Pack the same weight of cheese into a cup either way, and the microplaned version fills noticeably more of it, because a good share of what's in that cup is space between curls rather than cheese. That's why a recipe measured by the cup after microplaning looks generous next to the same weight shredded coarser. The scale never changed. The shape of the cheese did.
Why a dull microplane can't be sharpened
A knife sharpens because its edge is one continuous bevel — a stone can ride along the whole length of it and restore the same angle. A microplane's cutting edge isn't one bevel. It's hundreds of separate teeth, each shaped by exactly how much metal the acid removed and at what angle they were pushed up afterward, and there's no flat face left to grind against once that edge rounds over. Rinse it, dry it so it doesn't rust, and treat dulling as the end of its working life rather than a maintenance problem.