This clown triggerfish loves belly rubs
This is John Trimble's beautiful and healthy clown triggerfish, who loves being scratched on the belly. You can tell how excited the fish is at the prospect of interaction because the clownfish changes to a darker color at the sight of John's hand while flaring out and batting its fins to garner attention.
As John suspects, the clownfish is likely using John's finger as a surrogate cleaner wrasse or shrimp; fish often display stronger pigments and flash their fins to attract cleaner organisms to them. Or maybe this clownfish just likes the attention. Whatever the reason, it's pretty darn neat.
PSA: Triggerfish bite hard (I know this through personal experience). Remember: their teeth are designed to crush crustacean, coral, and rock. If John has inspired you, just be careful. Not all triggers are as amicable. And of course, never force your fish into something it is not comfortable with.
An interesting observation about "tame" reef fish is that species with the most dangerous teeth are often the most pet-like (ie. allows physical contact with their owners). Besides triggers, we've also seen friendly eels, parrotfish, pufferfish, oscars, and flowerhorns. The likely explanation for this trend is fishes with powerful bites are less intimidated by people because they are able to defend themselves with one nasty chomp. Again, be careful.