Wake the dragon in order to get out of the gate level 21 by shaking the phone, after he left the dragon will breathe fire issued.
Arrange the pipes on the right side with the true, for instructions on how to compile the pipe you can follow the letters in the door.
After the pipes arranged properly at the gate level 21, the pipeline will emit water from the tap and put out the fire on the floor, then the dragon will appear again at the door.
Solutions that dragon away from the gate level 21 is you take a sword in inventory menu and touch the dragon several times until it disappears and the door will open.
Now continue to the 100 Gates Level 22 Explanation, or back to the 100 Gates Level 20 Walkthrough