A method for accessing the contents of an assembly that includes a cap in sealing engagement with a fluid-holding vessel. In the method, one or more rib structures on an outer surface of a plastic pipette tip engage the cap as the pipette tip penetrates the cap and enters the assembly. A fluid transfer apparatus operationally associated with the pipette tip draws a fluid from the assembly into the pipette tip. The fluid-holding pipette tip is then removed from the assembly.