Black and Tan = Winnar!
Black and Tan + Mason Jar = Phail!
Black (Guinness) + Tan (Wheat Beer, Shock Top) = Phail and Aids!
With the lip of the mason jar you are blocking the pour of the "Tan" allowing the "Black" Guinness to pour over the lip freely.
INTERESTING SIDE NOTE:
Most people who don't drink Guinness, do so because "it's to heavy." Most people drink Wheat beers stating that they are light and refreshing, especially during the summer.
Guinness floats on top of most beers because it's specific gravity is less than most beers. It may be dark in color, and rich in flavor, but it's a lighter beer (Budweiser = 148 calories, Guinness = 125, Shock Top = 171 ). It also has less alcohol by volume at 4.0% than Bud Light, Coors light, Miller Lite (all @ 4.2%) and Shock Top (6.0%) .
Bet you did not know that?