Kiss a girl on the forehead make her happy for a day.
If you give her anal you'll make her whole weak.
Forehead Kiss
Explanation
Explain Bear
Listen up, buttercup. The joke is all about the difference between a sweet, innocent kiss on the forehead and, well, a more intense encounter. The punchline is unexpected and kinda raunchy, which is what makes it funny (or not, depending on your sense of humor... which seems questionable).