When we light a candle, why does the flame go against the gravity (up)?
When a candle burns, the nearby air is getting heated. The density of the hot air is less than that of the surrounding air and hence this hot air moves up like a cork inside the water moves up because of buoyancy.
Now the cooler air and oxygen rush in at the bottom of the flame to replace the moving hot air. When this cooler is heated this too is heated up and replaced by cooler air at the bottom of the flame.
This creates a continuous cycle of upward moving hot air around the flame. This air pushes the flame also upwards. This gives the flame its elongated or teardrop shape.
Thus because of the earth's gravity the flame moves up. In outer space where there is no gravity, the hot air will not move up and air will come to the flame from all directions and hence the candle flame will be spherical instead of its elongated shape at the outer space where there is no gravity. Without gravity there is no up direction for the warm air to rise.