Our daughter has severe reflux; so much that we give her cereal in her bottle at feedings to thicken it. The thickened formula needs a slightly larger nipple opening. Nothing new there I'm sure for many people here.
Her twin brother still uses plain formula, and hasn't graduated to the thicker formula, therefore still the thinner nipple hole.
They're currently differentiated visually by a small letter, which is hard to read from clear silicone. We had an experiment with nail polish on one; that didn't survive a test wash in the dishwasher.
Is there anyway to mark silicone nipples in a non-leeching non-toxic way?