Crystalline Opal

Crystalline Opal is an important ingredient of H2OGfat, the liquid that keeps the heads in a jar alive. In 3011, Professor Farnsworth discovered its significant side effect. Consuming Crystalline Opal while it is mixed with the H2OGfat, causes its consumer to travel back to the era of the person's head it is protecting. The time travels lasts for a limited time, depending on the amount of liquid consumed and the amount of Crystalline Opal mixed in. For instance, a brief lick lasts for several seconds while a large amount can go up to 24 hours.

It is unknown what its side effect is if it is consumed before having been mixed with a person's head; or if the person is alive at the time of the consumption.

Desperate to clean his family's name, Farnsworth used all of it to travel back to the era of the American Revolutionary War.

Trivia

 * Interestingly, it also transforms the persons clothes, so they fit with the era.