Simplify the following text for a Grade 9 native English-speaking person.
Original: The oldest writings mentioning yoghurt are attributed to Pliny the Elder , who remarked that certain nomadic tribes knew how `` to thicken the milk into a substance with an agreeable acidity '' .
Simplified: The oldest writings mentioning yogurt were by Pliny the Elder , who said that some people knew how to thicken the milk into something which was sour but tasty .