I guess it's all because you conditioned your mind beforehand. We tend to follow patterns.

there are nodes, or connections between concepts in our "mind" based on associations we have made from our experiences. This is a part of the study of cognitive psychology. When there is a higher level of association between two concepts due to our experiences, there is a shorter "distance" or time between relating the two concepts. In this example, the target concept is milk, which has a close aural/auditory association with silk (they sound similar and thus are closely grouped in our "mind"). This is the same case with the association between the concept "cows" and the target, milk, however, this is more of a semantic (meaning based) connection. Both stimuli, cows and silk, are more closely related to milk than water, and are related to each other via the node/concept milk, thus priming the participant to answer milk. When primed, the target concept will be on the tip of the participants tongue even before the question is completely asked. Thus, without second thought, the participant answers milk instead of the obvious answer: water. Other similar questions are: what do u put in the toaster? You probably answer toast, which doesnt need to be toasted. The answer is bread. Where do you bury the survivors of a plane crash on the Canadian and U.S. border? What ever you come up with, the answer is that you do not bury SURVIVORS for obvious reasons. Spell spot four times... what do you do at a green light? You probably said stop... the answer is go (dont try this in a car). If a red house is made of red brick, a black house is made of black brick, and pink house is made of pink brick, what is a greenhouse made of? Not green bricks - glass to let the sun in for plants.