Teacher had a fish that floats in water. He floated in a 2litre bottle.
He had his own air tank (a biro cap, with no hole in the top, held on to the fish with a paper clip and elastic band). We had to figure out a way to get the fish to sink and then float again.
We tried turning the bottle upside down (couldn't do this in the sea though), shaking the bottle and holding it sideways. The way to get the fish to sink was to squeeze the bottle.
When you do this, the fish sinks to the bottom, and when you stop squeezing, the fish floats again. This happens because the air in the pen top helps the fish float.
He had his own air tank (a biro cap, with no hole in the top, held on to the fish with a paper clip and elastic band). We had to figure out a way to get the fish to sink and then float again. We tried turning the bottle upside down (couldn't do this in the sea though), shaking the bottle and holding it sideways. The way to get the fish to sink was to squeeze the bottle.
When you do this, the fish sinks to the bottom, and when you stop squeezing, the fish floats again. This happens because the air in the pen top helps the fish float. When you squeeze the bottle, the air in the cap gets squashed a little (it compresses). This means that the bottle fills a little with water instead. Now the fish hasn't enough air to keep him floating, and the fish sinks.
When you stop squeezing, the air in the pen cap expands again, and now there is enough air in the cap again so the fish can float. We also tried this with a diver and it works in just the same way.
The boat floated, but didn't move. When we put just one drop of washing up liquid at the back of the boat, the surface tension is broken and the boat shoots forward.
This was really impressive, but we could only do it once. (Actually, we did manage to try again, but only after the basin had been washed and dried to get rid of all traces of wash up liquid). We can't wait to try this in the bath!
We could get water to spill sideways. First of all we tried lots of things - pouring it out of the side or the back of the jug, using our fingers or hands to get the water to spill sideways. 
We managed best of all when we used a piece of string, and got the water to run sideways along the string.
The second time we tried this, we only took our hand away when the bottle was under water. This time the water stayed in the bottle. This is because the water in the bottle wanted to get out, while the water in the basin wanted to get in, so the water ended up going nowhere and stayed in the bottle. 
Surprise, surprise, the card stayed on the bottom of the pot without falling off. This is because the air on the bottom of the card is pushing the card up into the pot, and the water is trying to get out, so both end up going nowhere at all.
We got an acitmel bottle and put newspaper round it so that it looked like a volcano, and a tray underneath in case we made a mess. To make an ordinary volcano you need bread soda and vinegar, but we wanted a delux model so we used red food dye and wash up liquid as well. Bread soda and vinegar mixed together make a fizzy gas called Carbon Dioxide. This gas comes rushing out of the bottle. To make the foam red we used red food colouring, and for lovely bubbles we used a little wash up liquid as well. 
The third and final time we put the bread soda into a balloon and the vinegar in the bottle. When we stood the balloon up, the bread soda dropped into the bottle with the vinegar in it. The gas that was made blew the balloon up. It was exciting!
The powder and vinegar mixture began to fizz. We heard that when you mix vinegar and baking powder together, a gas called CO2 or Carbon Dioxide (bad air) is made. This gas is heavier than air, so it sits at the bottom of the glass. When enough CO2 is made, it pushes the Oxygen (O2 good air) up higher in the glass. Now that the candle can't reach the O2, the flame goes out.
We also learned that we breathe in Oxygen and breathe out Carbon Dioxide. Plants and trees do the opposite - they breathe in CO2 and breathe out O2. That is why it is so important to have lots of trees planted around us, and lots of trees in the world.