The Vowel Glide
Examples from outside English are used to explain the long O, the long U, long I and ü (yu), because in English there is a "gliding" tendency in pronouncing these sounds. For example, in the word toad the long O leaves a faint W sound at the end, as though it were spelt towed; and in food and in puke the long vowels also glide toward a W. The long I in bee also glides towards what I'm not sure.
These "glides" don't occur in the O of German Tod, or the U of Italian luce, or the I of Spanish vida, or ü (yu) of the French putain.