Channing Tatum's got brown hair ... but he had a real blonde moment in L.A. last night.

Tatum was gassing up his vintage pick-up when we asked about the head transplant research going on in Italy right now -- seriously, it's a real thing.

We asked Channing if he could put his head on any person's body in the world ... which body would he pick? It was a pretty straightforward question, but Channing's answer ... just listen for yourself.

And right when you think the situation couldn't get any dumber -- our camera guy takes it to a whole new level.

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