Chemistry Lessons...
...were usually quite boring, but one time all the classes were gathered into one classroom to watch the "chemistry experiments". We walked into the room, the experiment area carefully roped off, and sat behind the thick pane of glass that had been set up to protect us. On the table behind the glass was a large bowl of water.

With suitable showmanship, the chemistry teacher told us all about the reaction of Potassium to water, and the dangers involved. Eventually, he produced a pair of long tongs and dropped a small piece of potassium in the water.

Nothing happened.

It sizzled briefly, and then sank to the bottom.

The teacher looked rather annoyed at Nature making him look less impressive. He retreated into the store cupboard, and the next thing we knew he had a chunk of potassium about the size of a golf ball, and plunked it in the water.

The lump exploded in a flash of light, making an incredibly loud crack. When our eyes had cleared, we realised the crack had been the inch thick glass in front of us cracking in half with the explosion. Pieces of the bowl were embedded in the frame, in the table, in the ceiling...

Best chemistry lesson EVER.
(Sivvus trolley collision in the fruit aisle led to a jam, Thu 29 Jan 2009, 18:47, Reply)