A considerable number of verbs change the vowel "e" in the stem to the diphthong "ie", and the vowel "o" to "ue". This happens when the stem vowel receives the stress. These verbs are referred to as stem-changing verbs. Examples include "pensar" ("to think"; e.g., "pienso" ["I think"]), "sentarse" ("to sit"; e.g., "me siento" ["I sit"]), "empezar" ("to begin"; e.g., "empiezo" ["I begin"]), "volver" ("to return"; e.g., "vuelvo" ["I return"]), and "acostarse" ("to go to bed"; e.g., "me acuesto" ["I go to bed"]).