The Twins first began to prank people, only after they discovered they could switch places without people realizing. George had gotten in trouble for breaking some plates, and was put in time out. Fred, being as close to his brother as he was, offered to sit in time out for his brother (it was partly his fault after all). so George took the first half, and Fred the second.
Molly came to relieve her son from time-out saying, "Ok George, you're all done. Fred's in the garden playing." Fred left, feeling surprised his mother couldn't tell them apart. Once he told George on his discovery, the scheming and plotting were on.