Possible Clues:

• Expose piece of hair causing stoppage of flow in pipe
• Most of your hair and some of your hair may stop the flow
• It stops the flow of dry hair
• Bubble that obstructs the flow of liquid in a pipe
• First class to Cork changing around Limerick has compartment to contain pressure
• Eg, moon base entrance
• Sub compartment
• What's on your 'ead? A bit of it may be what's blocking pipe!
• Bubble obstructing flow