Often, on standardized, multiple-choice tests, those who are taking the test are provided with a scantron sheet on which answers are given as "bubbles" (circular areas). The testers must fill in the answer with a pencil by coloring the entire bubble of their choice. By doing so, the testers are "bubbling in the answer." The following image may clarify my answer even more: http://www.google.com/imgres?imgurl=http://s3.amazonaws.com/rapgenius/1219431638Y3s31z.jpg&imgrefurl=http://rapgenius.com/239873/Chris-webby-websters-revenge/Bubble-in-the-answer-if-you-wanna-test-me&h=320&w=216&sz=59&tbnid=F1EuCnBidSA9GM:&tbnh=104&tbnw=70&zoom=1&usg=__1QG_xFf8hWlynUlpQfVSrAyU1eo=&docid=Es0WTMuG8go8DM&sa=X&ei=zeWvUcmpM6nB4AOF04HgDw&ved=0CDQQ9QEwAQ&dur=317