The density of an object can be calculated by taking the object’s mass divided by its volume. The density of water is 1.0 g/cm3. Objects that have density greater than 1.0 g/cm3 will sink when placed in water. Objects with a density less than 1.0 g/cm3 will float when placed in water.
So … why doesn’t the demonstrator’s hand burn?
This experiment has a trick to it that relates to density. The methane bubbles do ignite, and the flame is a real fire. Just before the bubbles are placed on the demonstrator’s hand, he/she dips his/her hand in a bucket of water. Methane has a density that is less than that of water, so when the bubbles are added, they sit on top of the water layer. The water coats the demonstrator’s hand, and the methane bubbles burn above it. Below is a close up visualization of what really happens.