Parity Scan Trainer

Every position has a number (shown in the little badges). The grid's secret value is the XOR of all face-up positions — but you never do binary in your head. Instead, the value is built from a few odd-or-even questions, one per "bit."

Each bit owns a region of the grid. Look only at the face-up cards in that region: if the count is odd, add that bit's value; if even, add nothing. Do this for every bit and the sum is the grid's value.

Speed tip: don't count. Sweep the region and flip a mental switch — off, on, off, on — at each face-up card. Where the switch ends is the parity.

Learn mode walks you through one region at a time with highlights and running math. Read makes you produce the whole number. Catch the flip is the full trick: hold the number, one card flips behind your back, and the old value XOR the new value names the flipped card.