Remember when you were a kid and you had that puzzle which was made of muddled-up squares in a grid with one missing, and you had to shift them around one at a time until the picture was put together? I bet you had one. Anyway Unblock is only a little bit like that, but it's easily as entertaining.

The game is simple. As the Google Play page describes it the goal is to get the red block out of the board by sliding the other blocks out of the way. Each block can only move on one axis, and there's only one hole for the red block to exit through. It's up to you to detangle the blocks and shift them out of the way without causing more blockages. It feels like picking a lock in the sort of RPG that involves lock-picking (and it says a lot about me that I have a reference for that, but not for actually picking a lock. I've got no idea what that's like).

I'm not sure if there are multiple solutions for each puzzle. There are certainly multiple pathways to get to that solution, and often you'll need to set things up in ways that look like they're getting worse before they get better. That and the backtracking you'll do as you try various possibilities and dead ends mean some puzzles get solved in a few seconds whereas others can take you a lot longer.

It's to the designers' credit that so many workable puzzles can be made from such a simple set-up. The box you play in is just six-by-six, even at the more difficult levels. But there are so many slight variations on the formula that Unblock will keep you entertained for hours.

I say hours, but what I probably mean is minutes. Unblock isn't the sort of game you settle in with for an evening and plug away at until the early hours. Rather it's a game you dip into between classes, while your kettle boils, or during a quiet moment at work. You do a couple of puzzles, maybe half a dozen if it's a particularly slow shift. But those dips add up and pretty soon you've spent hours shifting around pretend bit of wood. It doesn't feel that time consuming though, and having worked my way though a week's worth of these little puzzles I'm still not bored. Apparently there are 4,000 all up, but at the rate I'm going I'll know for myself soon enough. Unblock is free, fun and addictive. Recommended.