On 100 Floors: Level 21, it may seem like a difficult stage if you don’t know the exact solution to the problem. There is a picture in the middle of the door and it has a big oval which is an eye. To beat 100 Floors: Level 21 you need to understand that this stage is a motion sensor stage which means it has nothing to do with the touch screen itself.
To beat 100 Floors: Level 21 simple tilt your device on a 90 degree angle where you will have it straight up then it will reveal an eye. If you can hold that angle for about 40 seconds the door will unlock. If your tilt is slightly off the eye on the door will shut and you will need to repeat the process.