Why do your lips don’t touch when you say “touch” and do touch when you say “separate”?

Your mouth has to make certain shapes to make certain sounds like the t in touch your lips pout slightly at the start of the word to make the sound needed, and the sounds in separate require your lips to be closer together. A lot of speech therapy is learning to move your mouth in the correct way to help create the correct sounds.

It’s literally just down to the sounds of the words and the letters used in them

Hahahahah

It’s the Ps fault!

Oh my goodness, that’s a good question. I have not actually given that any thought and been racking my brain for a while and still can’t come up with an answer.