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McDonald's restaurants are typically very busy, so don't hold up the line by being indecisive. If you know that you are going to take awhile to decide, stand off the side to look at the menu. Once you know what you want, you can get in line and approach the register. When your order your drink, be as specific as possible. Always use the proper name for the item you are ordering, and specify exactly how you want it. If you do not specify, you will receive your drink how it is traditionally made. For example, the default milk used at McDonald's is whole milk, and the default temperature for most drinks is hot. If you do not specify that you do not want those things, that is what you will get.  If you order a coffee, specify how many creams and sugars you want. If you want a flavor, explicitly say what flavor you want. If you want a certain type of milk, tell the cashier. Like most of McDonald's products, all hot and cold coffee drinks come in small, medium, and large. If you do not specify what size drink you'd like, you may end up getting a size that you do not want.
Know what you want before you approach the register. Be specific with your order. Tell the cashier what size drink you'd like.