According to the American Society of Anesthesiologists (ASA) fasting guideline for patients undergoing elective surgery, the 2-hour fasting period is suggested for clear oral fluid (including water, pulp-free juice and tea or coffee without milk). This guideline does not give any suggestions for proper volume of clear oral fluid intake. This study is a prospective randomized control trials in children aged 13 through 17 years who are scheduled for an elective upper GI endoscopy procedure in the Gastroenteral Procedure Unit (GPU) at Boston Children's Hospital. The participants will be randomly assigned into one of four groups: Group 1 will consume 3 ml/kg of clear fluid by mouth at 2-hour period before surgical scheduled time, Group 2 will consume 5 ml/kg, Group 3 will consume 7 ml/kg, and Group 4 will consume 10 ml/kg. The investigators plan to recruit 72 patients in each group and 288 patients for the whole study. Research team will collect patient's demographic data, vital signs, information about their EGD procedure. Then actual volume of the stomach content and acidity will be measured from the content that is suctioned from patient's stomach during upper GI endoscopy procedure. The investigators believe that the information from this study will help establish a comprehensive NPO guideline.