Chill one tall soda-fountain glass in the freezer for 10 minutes.

Place 1 1/2 cups ice cream in the bottom of the glass and press it down gently.

Tilt the glass and pour in the root beer slowly along the inside wall, pausing whenever the foam rises.

Set the remaining 1/2 cup ice cream on top once the foam settles below the rim.

Serve immediately with a long spoon and a straw.