Arrow keys or A and D to run, and Up, W or Space to jump. On a phone or tablet three pads sit beside the room: left, right and a wider jump button. Holding jump longer makes you jump higher.

Press into the wall and jump. The direction you are holding decides what the jump does: hold toward the wall and you climb it, staying close enough to catch it again and go higher, so a single wall is enough. Let go of the wall first and the same button leaps you away from it instead, which is what you want for crossing a shaft. Sliding down a wall also slows your fall, giving you time to line the next one up.

Probably not the timing. Leap gives you a short window to jump after stepping off a ledge, and remembers a jump pressed just before you land. If a jump is failing it is usually distance rather than timing: try running at the gap from further back so you are at full speed when you leave the ground.

Ground that gives way. It holds for about a third of a second after you land on it and then falls, so it is a step and never a place to stand. They come back when you respawn, so a failed attempt never leaves a room impossible.

Because that is the whole idea of world six. Ice does not slow you down - your top speed is exactly the same as on stone, so every gap is still clearable. What changes is friction: you accelerate far more slowly, you cannot turn on the spot, and letting go of the direction sends you sliding about ten times further than it would on rock. Ice rooms are about starting to slow down before you think you need to.

It is locked, and a locked door is drawn differently: it goes grey, iron bars appear across it, and a small key count sits above it. Somewhere in the room there is a glowing key, and it is deliberately never on the direct route. Pick it up and the door lights green. One room asks for two.

They are the one-off rooms. Instead of a world number the badge names the rule that has changed for that room only: DARK, NO GRIP, LOW GRAVITY, HEAVY or WIND. There are five in the whole game and each rule happens exactly once, so none of them ever becomes routine.

No, and that is on purpose. Every room in the game has been verified as completable by an automated search before shipping, so being stuck always means there is a route you have not found rather than a broken level. The shop only sells skins - nothing in it makes a jump easier.

Thirty-four, across eight worlds, including the five one-off rooms. The worlds run in order and never go backwards, so a mechanic you have learned is never re-taught later.

Yes, automatically in your browser. You come back to the room you reached, with your coins and skins intact.