Puzzle games can be wonderful things when they are done right, because either you'll be playing it for far too long and be wasting days with it, or you'll have played it for half-hour and got distracted by something else. Of all (well, three) puzzles games I've played, Zoo Keeper falls into the former camp because of the variety of game modes and it is genuinely good fun and challenging.
The gameplay is simple; move-timer decrerases steadily, match at least three blocks (which are animal heads) in a row and they disappear, move-timer increases slightly, more blocks drop down, proceed through the game by reaching the quota for each level, get points, put name on scoreboard, gloat at previous 1st place player. Repeat ad infinitum. For each level, one of the animals provides double-points - the best way of racking up points. There are five game modes: Normal (with mandatory wierd storyline for puzzle games) which sees you play up to level 20; Tokoton makes you progress only by collecting 100 of each animal; Quest has 10 stages of different challenges to test yourself; Time Attack gives you six minutes to show off your skill; and 2-player battle (rarr!).
The concept is simple and it can be played without thinking - just use the stylus and switch adjacent animal blocks to get three-in-a-row. Just keep an eye for those sneaky, hiding animals and you'll be fine. Or use one of your stocked up handy binoculars that highlight all the possible moves. Or just panic and not get anywhere. Or look out for other moves before your current three-block disappearance is complete. Or plan your moves ahead of time. The sound of chained disappearances gives a sense of being 'in the zone' and makes you want to keep that sound going, all of which sounds impressive to anyone else.
So what, you many ask, is so special about Zoo Keeper - it just sounds like Bejewelled. Well, I suppose there isn't anything special in it, but it implements a good puzzle formula with a slight twist of achievement levels. It gives you immediate fun - the 6-minute challenge is perfect for when you are waiting for someone. The return of old skool scoreboard gives it that replayability factor. It's just pure and simple, unadulterated fun.