A 4D movie is a combination of a 3D movie with things like smell and touch (physical effects). These effects happen at the same time the movie is running. (Note: 4D movies are not geometrically 4-dimensional.) Because these physical effects can cost a lot of money, 4D movies are usually shown only at special places, like theme parks and amusement parks. However, in Thailand, Mexico, Hong Kong, Nepal, South Korea, India and the U.S., some movie theatres can also show 4D movies. The movie "Avatar" was one of 10 movies that have been changed to 4D. The first was "Journey to the Center of the Earth".