Definitions of tube:

• noun:   conduit consisting of a long hollow object (usually cylindrical) used to hold and conduct objects or liquids or gases
• noun:   electronic device consisting of a system of electrodes arranged in an evacuated glass or metal envelope
• noun:   (anatomy) any hollow cylindrical body structure
• noun:   a hollow cylindrical shape
• noun:   electric underground railway
• verb:   convey in a tube

  Example: "Inside Paris, they used to tube mail"

• verb:   provide with a tube or insert a tube into
• verb:   place or enclose in a tube
• verb:   ride or float on an inflated tube

  Example: "We tubed down the river on a hot summer day"