Brainsnag Level 13 answer: Upside-Down Key
The level says: “Open the padlock”
Hints first (no spoilers)
Hint 1Look closely at the key. Something is off.
Hint 2Keys go in teeth-down. This one is not.
Hint 3Rotate the key 180° with two fingers (or scroll the mouse wheel over it), then drag it into the lock.
Show the full solution
The key is upside down. Twist it with two fingers (desktop: scroll the wheel over it) until the teeth point down, then drag it onto the padlock.