When I was teaching, we looked at drops of blood under the microscope. They could see the different types of blood cells and how they behaved when you added a grain of salt or drop of sugar. For most students it was fun or and interesting. I had one student warn me that he fainted when he saw blood. He wasn't kidding. I had a whole room full of students and I'm afraid I didn't take him seriously enough. A little while later...sure enough...he fainted and was lying on the floor. Fortunately he didn't hit his head on anything on the way down. Maybe he did and that was the toughest part of his anatomy. smile