The competitive version offered by scala of malaysia has a problem. It doesn't work.

If players conceal information from each other, some will get a few pairs, some no pairs, some more pairs and when time runs out, the no pairs player will be clueless and just guess. The others with a balance of pairs will also not have enough information to make a reasonable guess.

Forget the competitive version. Let memory, logic and good feelings prevail!

This is a cleverly-designed cooperative version of Concentration, mixed with a touch of Clue. Three tiles are randomly set aside at the beginning of the game. Players try to form matches from the remaining tiles with the ultimate goal of determining the missing pieces before 12 clock tiles are turned up.

Secret Door is one of the best cooperative games I've seen. My children certainly love the way we get to work together as a team when playing this game, so this has become one of our favorite Concentration variants.

There are a few things to be aware of before purchasing the game, however. First, the game IS a Concentration variant, so if your kids have already played a lot of Concentration, this game will not feel fresh. Second, the game is published by a small publisher, and the game components look and feel like a homemade game. In particular, the tiles are made of a fairly flimsy card stock, and aren't likely to hold up to any abuse. The tiles must be punched out before the first time you play, and it is very difficult to punch out the pieces without making some sort of crease or tear on the back of the one of the pieces.