The Miracle (Synsepalum dulcificum) fruits are a small bright red, ellipsoid berry approximately 2 to 3 cm long and containing a single seed.
Miracle fruits are relatively a tasteless berry, but they has an amazing side-effect; After eating a miracle fruit, all sour things will instantly taste sweet. Eating even the sourest of lemons, one will taste only sugary sweetness; and the sweetening effect lasts for about 30 minutes or an hour.
Miracle fruits are native to West Africa. This plant is commercially not important as a food crop. Some attempts to exploit the striking effect on perception of making as artificial sweeteners have not successful.
from a nytimes article: A Tiny Fruit That Tricks the Tongue.