I've always found classical Newtonian physics, the Special Theory of Relativity, and (to a lesser extent) the General Theory of Relativity to be understandable at an intuitive level, but quantum phenomena just baffles me, even when I think I "understand" it.
PS. IvoDankolov: I find it difficult or impossible to think intuitively about quantum phenomena.
PS. IvoDankolov: I find it difficult or impossible to think intuitively about quantum phenomena.