What you need: 

Lighter 
Candle in a glass 
Empty glass 
Backing soda or Kaisernatron 
Spoon 
Vinegar 

How the experiment works: 

1. Place the tea light in a glass and light it. 
2. Fill a teaspoon with baking soda or Kaisernatron and carefully fill the soda into the empty glass. 
3. Carefully add vinegar to the soda.
4. Now hold the glass with the vinegar baking soda mixture over the candle and pretend to pour it out without letting any liquid run out of the glass.  
 

What did you observe? 

What happened:  A candle can only be lit and burn, if three things come together: Wax and wick, heat and oxygen. The liquid wax rises the wick and the heat of the flame splits the wax particles (pyrolysis). Now the oxygen comes into play, which reacts with the very reactive fragments. Carbon and water are formes and energy is released in the form of light and heat. The created carbon forms small soot particles which glow in the flame and then burn to carbon dioxide in the middle very hot part of the flame. If the flame is cooled down at this point by blowing into it or by wind draught, the candle will soot

Explanation: Soda or sodium hydrogen carbonate (from the baking soda) reacts with vinegar to form carbon dioxide (CO2). Carbon dioxide is not combustible and because it is heavier than air, the glass fills up slowly. If you now "pour" the glass over the candle, the carbon dioxide displaces the oxygen around the flame and thus prevents it from continuing to burn.