4H pencil

The Tenth Doctor used a 4H pencil and a mug to demonstrate how the Huon particles in the heart of the TARDIS—the mug—were attracted to the identical particles in Donna Noble—the pencil. When Donna, shocked, questioned, "I'm a pencil inside a mug?" the Doctor joked, "Yes, you are. 4H — sums you up." (TV: The Runaway Bride)