Heidi Klum's Magic Helmet Head

Heidi Klum performed a supermodel magic trick Wednesday ... riding bitch with an extremely large helmet and then removing it ... only to reveal a perfectly coiffed head of hair.

Heidi was riding with her bodyguard-BF Martin Kristen in Venice, CA when she showed off her hat trick.

They've taken a serious step in their relationship, wearing similar get-ups, including hipster sunglasses.