This was a while ago but still great. It was first hour at school and I was doing my very first science demonstration ever. I was trying to show my students what their blood is made of. I used corn syrup to represent plasma, then red hot candy to represent red blood cells, little red hots to represent platelets and so on. When you heat it up a little and mix it together it turns into a red substance and they can see why their blood it red. I started to heat it up and when I went to add more stuff it exploded from the bottom. The candies stuck to the burner and the classroom soon became very smoky. Luckily my classroom does not have a smoke detector or I would have set the fire alarm off for the whole school. The good news is that the students thought is was awesome. 
