Dax Shepard -- What's The Big Frigin Difference?!

Even though the temps are dropping, Dax Shepard is comin' in hot n' heavy! Strap in and see if you can find the sneaky switches in the two photos!

Earlier this week in L.A., the Hollywood hunk was spotted showin' off his tatted muscles while getting ready for his appearance on "Jimmy Kimmel Live!" with his boo, Kristen Bell.

Hang in tight, now, because it's time to test your skills and find the changes!