Thomas "Tad" Lincoln III was the fourth and youngest son of Abraham and Mary Todd Lincoln. The nickname "Tad" was given to him by his father, who observed that he had a large head and was "as wiggly as a tadpole" when he was a baby. Tad Lincoln was known to be impulsive and unrestrained, and he did not attend school during his father's lifetime. He had free run of the White House, and there are stories of him interrupting presidential meetings, collecting animals, and charging visitors to see his father. He died unexpectedly of an illness at the age of 18 on July 15, 1871, in Chicago.