Grin from Ear to Ear

Meaning:
To smile happily.

Example:
On hearing the good news, Jenny grinned from ear to ear.

Did you know?
When you are happy, you smile. And when you are very happy, your smile is so wide, it stretches from almost ear to ear.

Reference: http://dictionary.cambridge.org/dictionary/british/grin-smile-from-ear-to-ear