The Extrusion Reflex
When your baby pushes solid food out of their mouth using their tongue, it's called the extrusion reflex (it’s also referred to as tongue-thrust reflex). While it may seem discouraging that your baby doesn’t want to try new textures, this reflex is a primitive instinct to protect them. Since their body is still developing, the extrusion reflex protects them from choking on or aspirating foreign objects in the first few months of life. That way, if something accidentally enters your baby's mouth, the reflex would push it right back out again. The tongue-thrust motion ensures that breast milk or formula are the only things going into your baby's tummy. The reflex won't work on the breast or bottle, but they might engage it when you offer a spoonful of solids.
You can check to see if your baby demonstrates signs that they’re ready for solid foods, specifically by checking on the extrusion reflex. To check, place your finger on your baby’s lips and see if they push at the finger with their tongue. If they push on your finger with their tongue, they may not be ready for solid foods as their reflex may still be in place.