Teenagers and/or pregnant wives.
1.) They have rapidly changing mood swings. They are happy one minute, then angry the next.
2.) You never know what they want.
3.) They are difficult to communicate with.
4.) They eat more than anything I have ever seen before (and are hungry only an hour or so after dinner).
5.) They are extremely emotional.
If I think of some more things, I'll add to the list haha :)