diff --git a/1000-hours/sounds-of-american-english/2.1.5-ə.md b/1000-hours/sounds-of-american-english/2.1.5-ə.md
index 5ff49835..e92f6e1f 100644
--- a/1000-hours/sounds-of-american-english/2.1.5-ə.md
+++ b/1000-hours/sounds-of-american-english/2.1.5-ə.md
@@ -18,9 +18,9 @@ Since the tongue's movements are invisible during speech, it's difficult for lea
 
 ![articulator-tongue-tip-ɚ](/images/articulator-tongue-tip-ɚ.svg)
 
-以下是另外一个角度的图示：
+以下是另外一个角度的图示[^1]：
 
-Here's another illustration from different perspective:
+Here's another illustration from different perspective[^1]:
 
 ![retroflex-bunched-r](/images/retroflex-bunched-r.svg)
 
@@ -111,3 +111,5 @@ Keep in mind that in fluent, natural speech, many unstressed vowel sounds in syl
 	- word <span class="pho alt">wɝːd</span> <span class="speak-word-inline" data-audio-us-male="/audios/us/word-us-male.mp3" data-audio-us-female="/audios/us/word-us-female.mp3"></span>
  	- world <span class="pho alt">wɝːld</span> <span class="speak-word-inline" data-audio-us-male="/audios/us/world-us-male.mp3" data-audio-us-female="/audios/us/world-us-female.mp3"></span>
  	- worse <span class="pho alt">wɝːs</span> <span class="speak-word-inline" data-audio-us-male="/audios/us/worse-us-male.mp3" data-audio-us-female="/audios/us/worse-us-female.mp3"></span>
+
+[^1]: This illustration comes from https://globalspeechtherapy.com/how-to-teach-the-r-sound/