Lemon

The lemon is an item found in Paper Mario. It can be obtained by going to the Dry Dry Desert's oasis and hitting the western (left side) tree with Mario's hammer. Leaving the oasis area and returning will allow the player to collect another lemon from the tree, and this may be done repeatedly. They restore one HP and two FP and may be cooked by Tayce T. to make a Tasty Tonic. Lemons are also good for making the Lemon Candy, which is useful in bribing the Anti Guy in Shy Guy's Toy Box. Lemons do not fetch many coins when sold and are largely only good for cooking.

The lemon may be used as a way to earn the Pulse Stone which will grant Mario entry to Dry Dry Ruins. Using the lemon to obtain this item involves giving one to Sheek. Doing so will cause Sheek to tell Mario how to "meet" Moustafa, who owns the Pulse Stone. This is done by purchasing items in a specific order inside the Little Mouser's Shop in Dry Dry Outpost. It should be noted that it is not necessary to use a lemon to gather this information, and if the player already knows the order of items to purchase, the combination may be entered successfully without ever speaking to Sheek.

Donkey Kong Jungle Beat
In Donkey Kong Jungle Beat, lemons appear at the end of every level in the Lemon Kingdom. After Donkey Kong reaches the end of that level, he munches through the whole lemon. This lemon is larger than Donkey Kong himself.

Trivia

 * Lemons are mentioned in Lumpy's diary in Paper Mario: The Thousand-Year Door, where he states that the last revision of his foodstuff during his Dry Dry Desert expedition consists of Lemons and Limes.