Pour milk, sugar into quart-sized (.94 L) Ziploc bag; shake.
Pour ice and rock salt in gallon-sized (3.78 L) Ziploc back; shake.
Put small Ziploc into larger Ziploc. Slosh the bag back and fourth across your countertop until the ice cream becomes nice and thick.
This is Ziploc ice cream; grab a spoon and just eat it right out of the bag! ;]