Why do movies where adults play kids (Eddie Murphy in kid my buddy, etc) are funny, while movies where kids play adults are not.

That's all. <br><br>Edit: it seems like there were some confusion. I don't think that movies where kids play adults are not always unfunny. But there is always something off when an adult role is played by a kid.<br> Edit 2: I have to get ready for work, will give you some feed back this evening. Thanks so much for all comments.