I believe it is fair to say that Big, starring Tom Hanks, is one of the better examples of the “age swap” genre of films. You know the ones, where a child ends up swapping bodies with a grown up, quite often their mother or father, after making a wish near some kind of magical artifact. Whilst these films can be fun, they are often quite formulaic.
Big was slightly different though, and all the better for it. It may still have featured a magical object (in this case a fairground fortune telling machine called Zoltar Speaks) and a child making a wish to be grown up, but in this case the child doesn’t swap bodies with anyone else. Instead, they wake up the next day to find themselves fully grown.
The child in question in Big was a lad named Josh, who suddenly becomes a 30 year old man with the mind of a 13 year old. Josh first runs away from home when his mother thinks he is a kidnapper who has taken her son, and having nowhere else to go ends up at his school where he manages to convince his best friend, Billy, that he actually is Josh.
Billy helps Josh to get a job at a toy company as a data entry clerk, but it isn’t long before he befriends Mr. MacMillan, the head of the company and gets promoted to an executive job as a toy designer! This all happens because Josh happens to be talking to the boss one day in a toy store, when he comes across a giant floor piano, and in a very memorable scene Josh and Mr. MacMillan play Chopsticks together on the keyboard. Apparently the pair really did play the tune for real, as is evidenced by the very occasional wrong note.