A word search hides a list of words in a grid, then fills the empty squares with other letters. Scrutix chooses those filler letters so that they never spell a real word.
The problem with random letters
Most word searches fill the empty squares at random. By chance, random letters spell real words. You find one, you mark it, and the game refuses it without saying why. These are ghost words: real words that are not on the list.
Every filler letter is checked
Scrutix fills the grid one square at a time. For each square, it picks a letter that does not complete a word of four letters or more, in any of the eight directions. It checks with the full dictionary of the language, not only with common words.
Before the grid is shown, Scrutix reads it again in every direction. If a real word that is not on the list is found, the grid is thrown away and another one is made.
Fewer words rather than a false grid
Sometimes no letter fits a square. Scrutix then tries again, and if needed it removes one word from the list. You get a grid with one word fewer, never a grid with a ghost word. The first time you open the app, a card says it in one sentence: One promise.