The game doesn't just occur on one two-dimensional plane either – there are three planes of depth that your sackboy can move between. Most of the time your sackboy will automatically move from one plane to another as you travel along the level, but it's also possible to control which plane you move along too.
This is quite important, as bonuses are often hidden on the furthest plane behind pieces of scenery, so if you are even slightly obsessive about getting all collectibles you'll need to bear this in mind.
As well as interaction with your surroundings to consider, there is also the matter of physics to take into account. Thankfully, the physics model isn't entirely accurate and is toned down to make gameplay more enjoyable, but items around the world still bounce, wobble and teeter in an almost realistic fashion.
Physics is not just there to show off, either – levels are fashioned around the use of momentum, springs and other bouncy things rather than any athletic prowess on the part of your sackboy or pixel-perfect jumping skills as with more traditional platform games.
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Talking of levels, there are over twenty levels with eight different themes. Some levels are cruelly devious and take some time to complete, so there are hours of gameplay here even if you're just playing on your own. Each level is split into a number of sections, denoted by large glowing rings, and you only have a few lives – use all these up and you have to restart the whole level. Thankfully, your lives are reset each time you pass a new section marker, so only on the really tricky sections are you ever in danger of having to restart.
If you have a few friends over, you can play through all of the levels with up to three other people. Most levels also have specific areas that can only be completed in co-op mode with two or more players – there's a sign which tells you how many players you'll need to complete that section. Co-op play is a frantic free-for-all which usually consists of everybody running around to try and collect as many points orbs as possible before they actually have to co-operate to progress through the level.
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It's great fun, but since the screen stays focussed where most of the sack people are, it can get a bit crowded and it's not possible to run on ahead to get more orbs, as you'll get dragged back to the rest of the crowd if you wander off the screen. Even if you don't have any real life friends, you can still play co-op mode with your online friends – or even strangers, for that matter – so you needn't worry about feeling lonely for long.
Co-op mode is also a drop-in/drop-out kind of affair, so your chums can enter and leave a game when they want. Obviously this can be a bit awkward if they leave you in the middle of a co-op section of a level, so you might want to stick to playing with people you know...