Professor Farnsworth reasons that this time travel effect is caused by the rare powdered crystalline opal used to make the fluid, which keeps the heads alive in a temporal bubble.
He dumps a vial containing the world's entire powdered opal supply into Washington's jar and licks his head, transporting himself, Fry, Leela, and Bender back to colonial-era New York in 1775.
They capture David just as Revere begins his ride to alert Lexington of the imminent British attack that would start the American Revolution.
However, Fry unwittingly takes one of the two lanterns hanging at the Old North Church to burn the forged money, causing Revere to wrongly warn of the British attack "by land" rather than "by sea".
The four are suddenly sent back to 3011 and find that history has been altered: Great Britain has won the Revolutionary War and taken over all of North America, turning it into "West Britannia".