Thank you for your help.
How do we use precise scientific terms relevant to PSLE Science to explain? Does a bigger hole make it easier for the milk to flow out?


http://farm4.static.flickr.com/3032/3471323833_d5d51de0c9_o.jpg

How about this?

Air in the atmosphere is trying to enter the can through the same hole the milk is flowing out hence explains the difficulty of the milk flowing out of the same 1 hole.

In response to your question "Does a bigger hole make it easier for the milk to flow out?" , the answer is no based on the reason given above. The solution is to have another hole for air to rush in thus forcing the milk to flow out faster from the other hole.

What about the air in the can has the same pressure as the air on the outside of the can (atmospheric pressure) at the start. As the milk comes out, the air inside increases in volume since no more air is going in. As a result the pressure of the air decreases to the point where the pressure on the inside becomes lower then the pressure on the outside.

In addition to Bing's answer, there is also no displacement of the milk in the can unlike a situation where there are 2 holes.