How do you snc a movie onto ipod

Hello,

i have an ipod touch and i plug it into a mac, i also have a lot of music. i want to put a movie on my ipod and its in my library on itunes so i clicked sync movies and a message popped up saying are you sure you want to do that it will remove and replace all exsisting music moies and tv shows and be replaced by the movie. i dont want that to happen what do i do?