Two sounds close enough to trade places: the “th” of this and the “z” of zoo. Swap one for the other and a different word comes out — this page is how to keep them apart, in both directions.
Move your tongue tip from behind your teeth to lightly between them, keeping the voicing on.
“Your buzz is beautiful but the tongue hides inside! Bring it forward, between the teeth. Same buzz, new address!”Nunzio, when he hears it
You cannot hear your own swap. That is the whole problem.
Nunzio records you reading a sentence, scores every sound in it on your phone — nothing is uploaded — and tells you which of these two actually came out. Nunzio — Accent Coach is coming to the App Store and Google Play.