He is interested in the shape of a polyhedron.

This mirror is convex in shape.

His forehead is very prominent.

Her forehead is very attractive.

This mirror is made with a convex lens.

We need to think about this problem in multiple dimensions.

He made a model of a regular tetrahedron.

My family consists of four people.

I drew a four in mahjong.

His actions always have a duality.