Jimmy Fallon continues to excel on 'The Tonight Show,' and recently he sang a great doo-wop duet with Billy Joel using an iPad.

The app is called and it allows you to do multi-layered harmonies. Watch Billy Joel and Jimmy Fallon perform the Tokens' classic 'The Lion Sleeps Tonight.'

What is the App Jimmy Fallon Used on the Tonight Show to Sing with Billy Joel?

We found this app called Loopy that creates music by looping vocals or beatboxing.

Here's how Loopy describes the app:

Create music by layering looped recordings of singing, beatboxing, or playing an instrument with a savvy, sophisticated, tactile looper.