Actions: Turn and press the childproof cap (you can feel it goes down a little and is engaged to the slot of the childproof mechanism), tighten it. 

Tips:  Besides all the silicone rings sealing the junction of different components of the tank, one of the key factors to help to prevent leakage is the lower pressure inside the tank.  
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By doing it, this low pressure creates a differential pressure against the atmosphere and holds the liquid, preventing them from leaking out from the wick. 

Tighten the cap every time after the refill. if the cap is screwed tight, you should see the cap is smaller.

1#  Cause: Physical damages
2# Cause: Atomizer not in proper position
4# Cause: Residue / Condensation going out
5# Cause: The liquid is "shaken" out