Barack Obama says he’s decided on a running mate, but he won’t say who. The Democratic presidential candidate told USA Today on Thursday that he went with someone who is independent and would challenge him in the White House. He also said he wanted someone who is prepared to be president and would help him strengthen the economy.

He wouldn’t say whether he’s informed his pick yet. He says he won’t reveal anything else about the decision until he announces it.

Obama is planning to do so in a text message to supporters sometime before Saturday afternoon, when he’s scheduled to appear with his pick in Illinois. Asked by an Associated Press reporter when the text would be sent, Obama just grinned and said, “Wouldn’t you like to know?”