Einstein's liquid was a flask of green, viscous liquid created by Albert Einstein, who thought it was bionic fusion liquid for his time machine. After Einstein and his liquid were accidentally transported aboard the Doctor's TARDIS, the Eleventh Doctor informed him he was wrong. Einstein resolved to conduct more tests on it, but before he could do so, the liquid bubbled from its flask, splashed onto Einstein's face, and transformed him into or transposed him with an Ood.
The Doctor used an energy field generated by the TARDIS to change Einstein back and brought him home to 18 September, 1945. After Einstein left the TARDIS and the Doctor resumed his travels, a small amount of the liquid remained on the floor of the console room, bubbling and moving on its own. (TV: Death Is the Only Answer)