Change of Gastric Fluid Volume After Oral Hydration in Fasted Objects

Observations of water in five different volumes (10ml, 50ml, 100ml, 200ml, 400ml) will be performed on different days. Subjects prepare fasting for investigation, at least 6 hour for solid and 2 hour for clear fluid, and then 10 ml or 50 ml or 100 ml or 200 ml or 400 ml of clear water in a pre-randomized order and then the cross-sectional area of the stomach antrum will be measured via ultrasound every 10-minute until the contents are no longer visible. During measurement, the stomach will be checked for contents supine position and then turned to the right decubitus.