I never knew about these trees

And since they grow in the United States, you'd think more people would.

Don't eat the fruit of, burn the wood of, take shelter from the rain under, touch the sap of, or otherwise interact with the manchineel tree. In fact, if you're anywhere near it, bow to it politely and address it as "sir." And in a quiet, deferential voice. From a distance.

ADDENDUM: My friend Lisa Stapp suggests an alternate name for the manchineel tree: "the Tree of the Knowledge of Good and Evil." Whatever you do, ignore any snake that tries to get you to nibble!