Vowel lip posture
Have a look at the vowel chart represented here and slowly read out the different words. Pay close attention to your lips. Do you realize how they are quite round when you say boot, but rather tight when you say beet?
Source of the picture: people.umass.edu
We call this feature of vowel distinction Roundedness. Roundedness refers to whether the lips are rounded or not. In most languages, roundedness is a reinforcing feature of mid to high back vowels, and not distinctive. Usually the higher a back vowel, the more intense the rounding. However, some languages treat roundedness and backness separately, such as French and German (with front rounded vowels), most Uralic languages (Estonian has a rounding contrast for /o/ and front vowels), Turkic languages (with an unrounded /u/), Vietnamese (with back unrounded vowels), and Korean (with a contrast in both front and back vowels).