[Senbonzakura]

I am trying to use the principle of Mariotte's Bottle for an overhead tank, and I am not able to make it properly.

Every time I insert a pipe through a hole made on the top part of the tank into the water within, the water enters the pipe up-to the level it is present in the tank. But in theory the water should not enter the pipe and that is what maintains the constant head.

Can you help me understand what I am doing wrong? I guess it is because the hole is not 100% airtight, but that does not seem to be the problem in all the youtube videos that demonstrate this method using a simple bottle and straws.