I use to have a post-op patient. He had nasal canula for oxygen therapy. One day at the beginning of my shift, I went to take his vitals . every thing is normal except his O2 level. His O2 level was less that 90. I am so suprised and wondered how come it was so low when Oxygen therapy was still provided. I checked the oxgen tank and the canula tube. They were all good. After a few minutes, I suddenly found that the two prongs which was supposed to be placed in the nostrils were not in the right place. They were under the the nostrils and almost touched the upper lip. Accordingly, the patient was not able to get the oxygen . I adjusted the prongs to the right position at once. And 15 minutes later I tested his O2 again. It was 94. I felt so relieved at that moment. The patient recovered very well in the following days. I think I used my critical thinking to make a decision and resolve a problem.