Visas can only be obtained outside of the United States at a U.S. consulate. The visa stamp placed in your passport is an entry document permitting you to apply for admission to the U.S. as an F-1 student. Depending on your home country, visas are valid for a variety of timelines. If your visa expires while you are in the U.S., you must obtain a new visa the next time you travel abroad and return to the U.S. There are exceptions to this rule for short trips to countries contiguous to the United States.