1Two things about flipping a coin
A coin clumps — and a streak is a run in a row
You only need two ideas. Watch each one:
A coin has no memory
Every flip is a brand new 50/50. The coin doesn't know it just landed heads, and it never tries to "even out." So heads can land again and again — the flips clump up.
A run is a streak in a row
HHHH is a run of 4 heads. The longest run is one thing we can measure. We're going to measure it — and it tells you more than you'd think.
2Two strips that both say "random"
The "spread it out" strip vs the "let it clump" strip
These two short examples show what a streak detector counts. Either pattern could occur in fair coin flips; neither proves who made it.
The "spread it out" strip
Tidy and even. It keeps switching — H T H T H H — and almost never lets the same side land many times.
The "let it clump" strip
Messier. Somewhere in here is a surprising long streak of the same side — it almost looks like a pattern.
3Your turn — fake it yourself
Tap out a strip that looks random
Tap Heads and Tails to build a strip of about 40 flips — try to make it look really random, like a coin would. The meter tracks your longest run as you go.
Your "random-looking" strip
4Now run the streak detector
Your strip vs a fresh coin simulation 🔎
The detector will compare your strip with a new set of simulated fair flips of the same length. The new flips are still hidden. If you made fewer than eight, we use the short-run practice strip.
Guess before you scan
Look at the run you made. The other strip will use fresh simulated fair flips. Which longest run will be bigger this time?