By popular demand (Hi Heather, we've missed you), here's the full write up of our germs experiment. Most of this is straight out of Horrible Science magazine, issue 6.
We used the petri dish and agar agar powder they gave us with the Horrible Science magazine. We also found another little pot with a lid. We rinsed out both dishes with boiling water, then divided them into four quarters, marked with a permanent marker.
We used an old baby bottle we found in our cupboard to measure out 120 ml of boiling water, and mixed this in a measuring jug with a teaspoon of agar powder and a quarter of a teaspoon of vegetable stock powder. We poured the mixture into a saucepan, brought it to the boil and simmered it for three minutes (stirring constantly).
We put this hot mixture into the dishes and let it cool down. Now I read it again I see we should have had the lids on while it was cooling, but in fact we didn't. It took about two minutes to cool - might be longer if you had the lid on. We knew it was cool enough when it had set into a jelly which didn't fall out when we turned the dishes upside down.
We got some cotton buds and wiped them on various surfaces. If they were dry surfaces we had to put a drop of water on the cotton bud first. We then wiped the germy cotton buds gently on the surface of the jelly. In section 1 of each dish we put germs from the loo handle. Section 2 was from a coin, section 3 from the inside of Pearl's cheek and section 4 from the inside of Leo's cheek.
We left the petri dish upside down (to stop water dripping into it) on top of a shelf in our kitchen and put the other dish in the freezer. Three days later the petri dish looked like this:
The one that had been in the freezer just looked like it was covered with ice crystals. When it thawed out, a lot of the water had come out of the jelly and it looked very strange, but I didn't take any photos of it. That bit of the experiment wasn't very conclusive. They did it with gelatine power on Mystery Hunters, maybe that's different stuff.