In the Netherlands, tapping your finger on the center of the forehead means "He's crazy!", whilst tapping your finger on the side of your head means "He's brilliant!"
Don't get confused! There are lots of misunderstandings amongst foreigners about these nuances in the Netherlands!
Did You Know?
In most countries, tapping the side of the head (around the temple) or twisting your finger indicates that someone is crazy - i.e. almost the exact opposite of what this gesture means in the Netherlands.