“Don't be so openminded your brain falls out.”
“Do not be so open-minded that your brains fall out.”
“Keep an open mind, but no so open that your brains fall out”
“It pays to keep an open mind, but not so open your brains fall out.”
“Keeping an open mind is a virtue—but, as the space engineer James Oberg once said, not so open that your brains fall out.”
“If you're too open-minded; your brains will fall out.”