Potato

Potatoes were a vegetable on Earth, often used to make other Earth foods such as chips.

The Doctor was once imprisoned in the Tower of London with Sir Walter Raleigh, who kept going on about the potato, which he'd discovered. (DW: The Mind of Evil)

The Sontarans were said to look like potatoes on several occasions. (DW: The Two Doctors, The Sontaran Stratagem, SJA: Eye of the Gorgon, The Last Sontaran)

In 2075, the First Doctor, John and Gillian used potatoes as makeshift weapons to defeat the space pirates on Anastas Thrax's space station. (TVC: The Hijackers of Thrax)

Behind the scenes
It is often stated that Sarah Jane Smith was peeling potatoes in Irongron's kitchen in The Time Warrior, but this has been debunked.