You could just get one of those mason jars, get in a bathtub, hold the jar upside down and fill it with water. Then fart into the water filled jar, the farts will replace the water. Put some small unrecognizable nicknack on the lid, put it underwater and cap it tight. That way when you take it out of the water the object is in the jar with the farts and it's sealed.


Hand the bottle to someone and ask if they can identify the object inside. No matter what they insist it is, tell them they are wrong untill they ask to open the jar. Allow them to open it and get a gooooooooooooood look at it.