Question

Why are we not able to drink water from a straw with a small hole?

Answer 1

When we suck, the pressure inside our mouth decreases. But pressure on water is same as atmospheric pressure. So, as fluids move from high pressure to low pressure, water moves from glass to our mouth.

When straw has a hole, and when we start sucking using it, the air from outside comes inside our mouth through the hole due to air pressure. So, we cannot create enough low pressure inside our mouth to lift water. So, water will not move from glass to our mouth.

Air is lighter than water. So, whenever we suck, lighter air moves inside easily. Water will not. If all holes are closed, then water will move again.