[I - <no name>]
Walk into the exit.
[II - What Goes Up…]
Walk across the gap. (don't jump)
[III - Move The Window]
Drag the game window (not the one you see on a ledge - the actual window the game is in) anywhere.
[IV - 5 Is Fake]
Move so you are directly under the exit door. Jump up, and at the height of your jump (judge it - you can't see yourself at this point) move right. Move as far right as you can, then do a high jump left.
[V - Turn Back Now]
Go to the left wall. As you touch it, hold right.
[VI - Backwards Thinking]
Jump up the large ledge using down, not up.
[VII - Hint]
The word Hint is actually a solid block - use it to get to the exit.
[VIII - Not What You'd Expect]
Move the lower movable crate left, so when you push the upper movable crate off the left edge it'll land on top of it. Then jump onto the two-block stack and over onto the exit.
[IX - Fooled You]
This level is evil - some of the flooring over the spikes can be fallen through. However, not much can - if you keep jumping right you will be OK. None of the flooring on the higher tier can be fallen through, however a certain spot about 64 pixels right of the exit will warp you back to the right of the higher tier. Hit this spot once to get an idea of its location, remember it, then jump over it as you touch the exit.
[X - R. Kelly Hit]
You don't have to be on the floor to jump - keep pushing up to get to the exit. (incidentally, the clue refers to the song I'm Flying Without Wings by the R&B artist R Kelly.)
[XI - Awareness]
Technically, it's not XI - it actually says H. So push H to finish the level.
[XII - What You'd Expect This Time]
Ignore the crates. Jump over the crate and the spikes, and get to the far right of the level. You can fall through the floor to reveal an alternative exit.
[XIII - I Lied]
Move right a bit to reveal the actual level. After that there's no tricks - simply jump across each block to get to the exit.
[XIV - Lose Focus]
Open a different window (or click another open window), then click the game window again. An exit will have appeared.
[XV - Start]
[XVI - Up And Over]
Hold down left click over the guy, and drag him over the big wall to the exit.
[XVII - Mice Know]
The mouse cursor will jump to a certain spot in the level - this points to an invisible block which you can jump onto to help reach the exit.
[XVIII - Think Small]
Minimise then maximise the game window to reveal an alternative exit.
[XIX - Boss]