Some words with the vowels [ʊ] ( book ) or [uː] ( smooth ) may be misunderstood if the vowel is not pronounced clearly. There are also word pairs that require a distinction between these two vowel sounds: Luke / look , pull / pool , and soot / suit , Fortunately, there are not many of such pairs, and for a few words, like hurray! and oops! , both vowels are acceptable. Some native speakers don’t make a distinction between [uː] and [ʊ] and are nevertheless understandable, but it is not advisable for learners to try this approach, if intelligibility is their aim. Pronouncing [ʊ] is the bigger challenge for most learners, while the pronunciation of [uː] is usually less problematic. The good news is that words with [uː] are much more common than are words with [ʊ].