Binary, hex and decimal. The way computers count.
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We count in tens for the accidental reason that we have ten fingers. Computers count in twos because a switch is either on or off. Hexadecimal exists because sixteen is a power of two and four binary digits fit neatly into one hex digit.
None of that is difficult once you see that a base is just a set of column values. In base ten the columns are 1, 10, 100. In binary they are 1, 2, 4, 8, 16. Everything else follows.
This version: Base eight and mixed conversions. Your best score is saved in this browser, so you can come back and try to beat it. Nothing is uploaded and no sign-up is needed.
Not strictly, but it is the clearest possible demonstration that our number system is a choice rather than a law of nature. Most people find that genuinely interesting once it clicks.
Learn the column values 1, 2, 4, 8, 16, 32, 64, 128 and add up the columns with a 1 in them. 1011 is 8 + 0 + 2 + 1, which is 11.
Because base sixteen needs sixteen digits and we only have ten. A to F cover the values ten to fifteen.