Really good puzzle games are ones which build their challenges around a simple core mechanic, using increasingly fiendish puzzle design, rather than complex rulesets to ramp up the difficulty. You see this principle in games like Cogs, where even the most elaborate contraptions are made out of sliding block puzzles, in Alcazar's increasingly intricate line patterns, and more recently in Noumenon Games' deceptively easy Snakebird. It controls like a turn based Snake, with movement restricted to an invisible grid, your snakebird retaining its shape after each move, and an inability to move backwards.
The aim of each level is similarly straightforward: make your way through the environment, collecting any fruits you find, before heading for the exit. These fruits aren't just an arbitrary requirement for finishing the level though; each one extends the length of your snakebird, allowing access to previously inaccessible areas. This increased length can be a double-edged sword though; poor planning can lead to your snakebird becoming trapped by its own, newly elongated tail. The biggest danger though is gravity, while a snakebird cannot be made to move into spikes or dive into the sea below, it can and will fall to its doom unless some part of it is on solid ground. Therefore the major challenge of the game is ensuring your next move won't leave it without some kind of support.
And challenge is the operative word, after two or three levels introducing its mechanics Snakebird quickly ramps up the difficulty, presenting you with levels which at first glance appear trivial but which take serious forethought to overcome. Later levels then start to introduce additional elements like movable blocks, teleporters, and multiple snakebirds in a single level who must shove, lift, and support each other to reach the exit. None of this changes the core mechanic though; all of these new elements follow the same rules, so even as the levels become increasingly complex there is a sense of consistency and fairness throughout. Thankfully Snakebird also includes a number of quality of life options so the experience is never too frustrating: the movement grid can be made visible and you can undo any number of moves right back to the start of a level.
Attention should also be paid to the audio-visual elements of the game, which create a relaxed atmosphere that helps to temper any frustration you will inadvertently feel while playing. Graphically levels are built from sprites layered in 3D along the Z axis, with an orthographic camera being used to give the illusion that it is in 2D. Flat, distinctive coloring is then used to visually separate solid objects from the background; though this is not always successful. The adorable animation on the different snakebirds themselves deserves particular attention, as this gives personality to what could have been fairly boring characters; the way they stare worriedly at spikes or longingly at fruit is particularly endearing. Finally the music, while a bit repetitive at times, really fits the atmosphere of the game perfectly.