(CNN) CNN's Anderson Cooper on Tuesday night expressed surprise that President-elect Donald Trump had the time to tweet despite the heavy burden of preparing for the presidency.

"When I first heard that he was tweeting about something that was on this broadcast ... I kept thinking, 'Doesn't he have, like, a briefing book on ISIS to be reading last night?' "

Cooper, however, went on to express joking gratitude to Trump for watching the show.

"There's a huge amount of information ... for him to be absorbing now," said Cooper, speaking on "AC360." "[I] appreciate he is watching the show. ... But what is he doing?"

Trump fired off a series of tweets Monday night and Tuesday attacking CNN for accurately reporting that his unsubstantiated claims of massive vote fraud in the election were incorrect.