This is an easy way to create a molotov cocktail which is not available to purchase or earn in the game and can just be used for fun or to show off to friends.

1) Use the plasmid "Incinerate".
2) Use the plasmid on a bottle of alcohol of any kind.
3) The top of the bottle of alcohol should set on fire.
4) Switch your plasmid to "Telekinesis".
5) Use the plasmid to pick up the flaming bottle of alcohol.
6) Find an enemy such as a splicer.
7) Throw the bottle at the enemy and it should blow up setting your enemy on fire.

This is the only method in the game to obtain a molotov cocktail and they are actually very powerful. They can be used to injure Big Daddies and make the fight easier.

Here is a Youtube video if you are confused by the tutorial:
(Skip the first twenty seconds)