Miracle Berry
The miracle berry, otherwise known as the miracle fruit, is a plant that produces berries. These berries are special because if you eat them, later sour foods then taste sweet (for example lemons). This effect lasts for half an hour to two hours.
These plants are found in West Africa. However they are available for purchase in freeze dried or tablet form, but are quite difficult to get outside of asia.
It is believed to work because the fruit contains miraculin, which binds to the tongues tastebuds. No one really knows how this causes sour foods to taste sweet, but the main theory is that it changes the shape of the tastebuds.