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Add 10 to 12 ice cubes and 1 tablespoon (16.5 g) of salt to the bag. It's important to use solid ice cubes instead of crushed ice or cubes that have hollow centers. The salt will lower the freezing point of the ice in the bag, which will make it even colder. This will help the slurpee mix faster. Put 1 cup (240 ml) of Coca-Cola into a sandwich-sized sealable bag and seal it shut. Try to remove all of the air before you seal it. Check the seal of the bag to ensure no soda is leaking. You don't need to remove the air from the bag with the ice before you close it. This will give the Coca-Cola bag some room to move as you shake it. The ice should be able to move easily around the bag of soda. Keep shaking until you see the Coca-Cola freeze and then break up into a slushy mixture. If your hands get too cold while you're shaking the bag, wrap a kitchen towel around the bag. Open the bag with the ice and lift out the smaller bag of Coca-Cola slurpee. Open the slurpee bag and pour it into a small serving glass. Stick a straw in the slurpee and drink it before it begins to melt. If you'd like to make a larger serving, make them in batches instead of doubling the amount of liquid in the bag. This will ensure that they freeze quickly.

Summary:
Put ice cubes and salt into a 1 US quart (0.95 l) sized sealable bag. Pour the Coca-Cola into a smaller sealable bag. Place the Coca-Cola bag into the bag with ice and seal it shut. Shake the bag for 3 to 5 minutes. Serve the Coca-Cola slurpee.