5 posts.
A microwave oven fills its box with a pattern of standing waves, and the hot spots are exactly half a wavelength apart. Melt chocolate in short bursts, measure the spacing, multiply by the oven's frequency, and you have the speed of light to within a few percent.
Add up what you actually need to keep running, convert it to watt-hours, then pick a battery and a panel that match. Loads with real numbers, an interactive sizing calculator, and the four things that make an outage plan fail.
Divide 72 by the interest rate and you get the years it takes money (or debt, or inflation) to double. Why it works, when it's off, and an interactive calculator that compares the shortcut to the exact answer.
Three field methods: the stick trick, the shadow method, and the trigonometry of a phone clinometer. Each needs only a tape measure, and there's an interactive calculator to check your numbers.
Binary is just place value with twos instead of tens. Learn to convert binary to decimal and back in your head, then flip the bits yourself in an interactive 8-bit converter.