Should you put lids on milk?

Of course! If you're not going to be drinking something for a long period of time, then put a lid on it and put it in the refrigerator. Leaving any time of perishable liquids or food out in the open without covering it up can cause bacteria to get into the cup. Germs are everywhere, even in your fridge. You can't predict what will happen if you leave things uncovered, even food. Bugs fly around everywhere. They carry diseases too.