Crystalline Opal

Crystalline Opal is an ingredient of H2OGfat, the fluid that keeps the heads in a jar alive. In 3011, Professor Farnsworth discovered it's a side effect: drinking it causes the person to travel back to the head's era for a limited time. A brief lick lasts for several seconds while a large amount can go up to 24 hours.

Trivia

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