Lego has created one of the most funny Minifigure heads in their new Lego Dimensions Adventure Time Level Pack. Finn the Human has a goofy smile, and on the back side a derpy smile. Reddit user ColmiYveul realized the potential that this piece had and combined it with other well known Lego Minifigures. As you can see the results are something to remember. Then they quickly put together a small series with Tolkien and Disney Minifigs.
If you want to make your own, the good news is that the Adventure Time Level Pack is new, and still in stock at most retailers. Who knew one piece could has so many uses.
You can find the Reddit page for ColmiYveul Lego Adventure Time stuff over here: https://www.reddit.com/r/lego/comments/5ca10x/im_pretty_sure_finns_head_now_is_my_favorite_lego/