When I was a teen, I watched my mom apply her foundation with her fingers and thought it was the way to go! But now I know better. (Sorry, mom!)
When you use your fingers to apply your makeup, you're not only getting your dirty hands all over your face, you're also contaminating your makeup with your dirty hands.
Use a makeup applicator, sponge, brush or any other tool to apply your makeup, and keep your hands away from your face.
