I've allways thaugt that Dr. Robotnik and Dr. Eggman were the same thing but some people tell me that Dr. Robotnik is Dr. Eggman's father or even grandfather. Which is true?
Dr. Robotnik is Dr. Eggman's old name - Eggman was his name in Japan, and in recent games they're using his original name, so it refers to the same person. However, Dr. Gerald Robotnik is Dr. Eggman's grandfather.

~ Crazy C Lea