Opposers of Wegener' s theory start pointing out mistakes and miscalculations in his theory. It was especially criticized due to the fact that Wegener had no real idea on why continental drift occurred. He had said in his book that the continents were plowing through the crust like icebreakers, and that centrifugal force or tidal forces were responsible for it. This idea was marked out when opposing scientists pointed out that those forces were not strong enough. Also, dues to some miscalculations, Wegener predicted that the continents moved about 250 cm a year, more than the amount they move this year. This greatly detoriated the validity of his theories.