/s/ 和 /θ/ 这两个音位，也是很多国内英语学习者容易搞错的。原因应该是因为中文普通话中没有 /θ/ 这个音位，所以很多人就用 /s/ 来替代 /θ/了。
图1：辅音 /s/ 和 /θ/ 的构音对比
从上面的图1可以看出，/s/和 /θ/都是摩擦音，但是这两个音阻碍气流的最小通道位置(成阻位置)不同。/θ/是上下牙齿轻轻咬住舌尖，或者说用舌尖把上下牙齿顶出一个通道，让气流从此通道摩擦着出去。而 /s/ 是舌尖和上牙槽之间形成一个狭窄的通道让气流摩擦着出去。当然这两个音都是清辅音，声带不振动。
下面列出一些分别含有 /s/ 和 /θ/ 的单词给大家练习区分这两个音：
| /s/ | /θ/ |
| sin | thin |
| sick | thick |
| sink | think |
| sank | thank |
| sum | thumb |
| sing | thing |
| sigh | thigh |
| sort | thought |
| mass | math |
| pass | path |
| mouse | mouth |
| force | forth |
下面列出一些含有/s/ 和 /θ/的句子供大家练习：
1. He is sinking when he is thinking.
[hiː ɪz ˈsɪŋkɪŋ wɛn hiː ɪz ˈθɪŋkɪŋ]
2. Jim is thinking when Jerry is singing.
[ʤɪm ɪz ˈθɪŋkɪŋ wɛn ˈʤɛri ɪz ˈsɪŋɪŋ]
3. The sum of your thumbs is two.
[ðə sʌm ɒv jɔː θʌmz ɪz tuː]
4. We can pass this path to the sink.
[wiː kæn pɑːs ðɪs pɑːθ tuː ðə sɪŋk]
5. The mouth of the mouse is full of food.
[ðə maʊθ ɒv ðə maʊs ɪz fʊl ɒv fuːd]
6. I think Jessie's sister is too thin.
[aɪ θɪŋk ˈʤɛsiz ˈsɪstər ɪz tuː θɪn]
7. Something sticky is on Cathy's thumb.
[ˈsʌmθɪŋ ˈstɪki ɪz ɒn ˈkæθiz θʌm]
8. I think something is missing.
[aɪ θɪŋk ˈsʌmθɪŋ ɪz ˈmɪsɪŋ]
9. The thief stole three sinks.
[ðə θiːf stəʊl θriː sɪŋks]