One word, a dozen times, with nowhere to hide
In an ordinary word search the random filler is on your side. A word starts where its first letter sits, and that letter is rare among the others, so your eye goes to it. A one word word search takes that away. There is one word, it is hidden eight, twelve or sixteen times, and every other square in the grid is one of its own letters too: BOOK ten times, in a grid of nothing but Bs, Os and Ks. The row beside the grid gives you the word and the number, and the puzzle is finding every copy.
The number is exact. Fill a grid at random from three letters and BOOK turns up by accident several times over, so the engine chooses the filler square by square, checking every line through each one, and never writes a letter that would complete another copy. Then it counts the finished grid in all eight directions and deals again if the count is off by even one. When the counter says twelve, there are twelve: not eleven and a mistake, not thirteen and a surprise.
Easy hides the word 8 times in a 10 by 10 grid, across and down only. Medium hides it 12 times in 13 by 13 and adds the diagonals. Hard hides it 16 times in 16 by 16, and copies may run backwards, so every line has to be read both ways. Two copies may cross or share a letter, but no two ever sit on the same squares.
The word comes from whichever topic the page deals you: a colour from Colours, a planet from Space. Words of four to eight letters are eligible: a three-letter word crowds its own copies until the count cannot be kept exact, and a nine-letter one will not fit an Easy grid eight times over. New deals another topic and another word.
Why your eyes can't skim it
Vision researchers draw a line between two kinds of looking. If one red dot sits among blue ones you see it at once, and it makes no difference whether there are five blue dots or fifty: the odd one pops out. But if the target is a combination, a red circle among red squares and blue circles, you have to check the items one by one, and the time it takes grows with their number. Anne Treisman's experiments in 1980 drew that line, and a one word word search is built on the slow side of it. Nothing pops out, because every square is a candidate; what you are looking for is an arrangement, and arrangements have to be checked.
So this is a search in the older sense, the kind you do for keys. It rewards a method over a glance, which is why people who are quick at an ordinary word search often find their first of these oddly slow, and why the last copy of twelve can take longer than the other eleven put together.
One word, written twelve times without a pencil
Finding GREEN twelve times is spelling it twelve times, and the attention it takes is the kind that makes a spelling stick. It suits a word of the week: open the topic the word belongs to, set Mode to One Word above the grid, and the puzzle picks a word from that topic's list; New picks another from the same topic. For a class, print it: the pack prints the word and its count under the grid and marks every copy on the answer key. Try colours for the youngest solvers, or space for longer words. The other kinds of word search here are on types of word search.