Notice that th is the first thing to vanish on a bad phone call. That is not your connection failing. It is physics, and it explains most of what makes this pair hard.
A Sound Most Languages Skip
The th in thing is rare across the world's languages. Korean, Japanese, French and German all manage without it entirely. When your ear meets a sound with no existing category, it does the sensible thing and files it under the nearest one available, which is almost always s. That is a reasonable decision, not a lapse in attention, and it is why the fix is building a new slot rather than listening more carefully to an old one.
Two Different Air Channels
Both sounds are made by pushing air through a narrow gap, which is why they land in the same family. What changes is where the gap is and how tight it gets.
th in thing
The tongue tip sits against or just between the front teeth, and air spreads out flat across a wide, soft opening. The result is dull and low-energy — closer to a leak than a hiss.
s in sing
The tongue tip lifts to the ridge behind the top teeth and forms a narrow groove down the middle. Air fires through that channel at speed, which is what makes s sharp, high and loud.
Listen for Sharpness, Not for the Word
Once you stop trying to identify the word and start judging the texture of the sound, this pair gets noticeably easier. An s cuts — it is bright and focused and sits high. A th is blunt and quiet and spreads out. This also explains something you have probably noticed: th is the first sound to disappear on a phone call or in a noisy café. It carries less energy, so it loses to background noise while s survives. If a word sounded like it had a soft gap where a consonant should be, th is the safer guess.
The Other Directions Th Drifts
- thin / tin
The same th, colliding with a stop consonant instead of a hissing one.
- thirty / dirty
Third direction for the same sound — this time it drifts toward d.