From the Super Mario Wiki
The Smile
The Item Shopkeeper will give a bright smile to the player.

First appearance

Wario Land 4 (2001)
The Smile in the Item Shop menu.

The Smile is an item that appears in the Item Shop in Wario Land 4. Unlike the other items, which are there to assist the player in defeating a boss and cost a certain amount of medals, the Smile is free, but is essentially a joke item. "Buying" the item only results is a smile from the Item Shopkeeper; a sound effect also plays each time the player selects it.