The answer to the joke "what do cows drink?" is water. The joke is a trick question that plays on the similarity between the words "silk" and "milk." The person being asked the question is first asked to say "silk" five times and then spell it. This is meant to confuse them and make them think of the first thing associated with cows, which is milk. Then, when they are asked what cows drink, they are likely to say "milk" instead of the correct answer, which is water. The joke has been around for a long time and has been used in various forms.