The Company It Keeps
A letter flashes inside a word, inside the same letters shuffled, and on its own. You are better at the one inside the word. The usual explanation is that you guessed it from the rest, so this page removes every scrap of guessing value and measures what is left.
Four characters flash for a few hundredths of a second, a pattern lands on top of them, and two letters appear under one position. You pick the one that was there. That is every trial.
The strings are of three kinds, mixed one after another: a real word, the same four letters with the probed one left where it was and the other three shuffled into an order English does not allow, and the probed letter alone between hash marks.
Both offered letters always make a word when the string is a word, so knowing English tells you nothing about which of them it was. That is the entire trick, and it is why this is a measurement rather than a demonstration.
About nine minutes, five short rests, nothing recorded, nothing leaves your browser. Sit at a comfortable distance and do not lean in, since the exposure gets set to your threshold in the first minute and then never moves again.
Every item passed the built in check that a scramble holds exactly the letters of its word.