Upon grabbing a glass with your fingers, the downward motion of the glass is halted by the frictional force, which is directed upward and related to the amount of force your fingers exert toward the glass (barely gripping a cold glass of water on a hot summer day will not be sufficient to prevent the glass from falling). Since the frictional force directed upward is proportional to the strength with which you grip the glass, why doesn't the glass move upward the harder you grip it?