/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.
2. Jim is thinking when Jerry is singing.
3. The sum of your thumbs is 2.
4. We can pass this path to the sink.
5. The mouth of the mouse is full of food.
6. I think Jessie's sister is too thin.
7. Something sticky is on Cathy's thumb.
8. I think something is missing.
9. The thief stole three sinks.