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The rule that lets you scale up a gas reaction - a bottle of gas and a cylinder of gas behave the same way.

For a welder, a battery charger, a lamp on a long lead, and any step-down or step-up. Power is roughly conserved, so a step-up gives you less current for the same watts.

Mould on the same corner every winter is not bad luck. Look for the cold spot with a thermal camera or the back of your hand.

Comparing variability between groups with different averages, such as two flocks.

Strain each wire to the same tension, lowest wire first, and keep the strainer run at the same place on every wire. See Fencing Wire And Straining.

A low power factor means more current for the same useful work. It matters for pumps and motors.

How long before a machine, shed or solar array pays for itself. Ignore interest for a quick check.

How fast work gets done. 1 hp = 746 W, so a 50 hp engine is 37 kW. A machine doing the same work in half the time has twice the power.

Grain heaps, fertiliser heaps, sand and gravel cones, and conical feed piles. A third of the cylinder of the same base and height.

The force needed to move a mass. The tractor pulling a trailer, the ramp lifting a load, a horse pulling a plough. Double the mass and you double the force, for the same acceleration.

Margin is measured against the selling price, markup against the cost. They are not the same number and confusing them is how people lose money. A 50% markup is a 33% margin.

The quickest way to plan a bed or a border. Onions at 10 cm square give 100 a square metre; potatoes at 30 cm give 11.

Mass is how much there is, weight is how hard it presses. A sack of fertiliser has the same mass everywhere and a different weight on the moon. Lifting power is usually quoted in kilograms, which is really a mass in disguise.

The right pilot hole lets the screw pull the two parts together instead of just splitting the timber.

Why a soft bag of sand and a hard block hit differently at the same speed, and why you roll with a fall. Doubling the speed doubles the momentum and quadruples the stopping force.

Only worth it if you grow one variety at a time, or if the neighbours are not growing the same crop. Otherwise you will save a hybrid.

The jack, the brake press, the loader bucket and the post hole rammers. A 10 cm piston moved with 100 N will lift 1,000 kg on a 1 m piston.

The belt and pulley rules for a lathe, a compressor or a sander. A small pulley on a shaft with a big one on the belt gives speed and torque.

The stiffness of a material. Steel is nearly twenty times stiffer than oak, which is why a steel frame sags less and lasts longer than a timber one of the same size.

Lime makes the maize release its niacin, which is the whole point of treating it. See Flint Corn And Popcorn.

For a gearbox, a quad final drive, a mower gearbox or a winch. A 3:1 reduction triples the torque and drops the speed to a third. The power stays the same, less the losses.

The quickest kitchen arithmetic. Divide by the factor for a smaller batch, and remember that a double batch does not cook in double the time.

Weigh the rice and the water and the result is the same every time. Salt the water, and let it rest before you cook it.

Warm the milk to 45 C, cool to 40 C, add the starter and keep it warm. See Yoghurt And Kefir.