I've just got involved with the Teacher Scientist Network (as a scientist) the idea is that scientists go into schools locally as a way of helping to make science seem interesting and real. I have been paired up with a class of 10 year olds - which is great, but i'm trying to think of fun things to show them and talk to them about (i'm a biologist) if anyone has any ideas i'd greatly appreciate them thanks!

Might be a bit too chemistry based for you but I've great success with getting children to do simple paper chromatography, they love it! Get some thick filter paper (filters for coffee machines work well if no lab ones handy), cut into strips (about 10cm by about 2cm) and with a (water soluble!) felt pen (i.e. blue) draw a small dot about 2 cm from one end. Dip the end nearest the dot in some water and prop the strip against something to keep it upright. The water (mobile phase) will travel up the paper (stationary phase) and separate the inks (i.e. into green and yellow). You can get some cool separations. Experiment with different pens/colours until you get something you know will be impressive! Best of all, it's nice and safe with no dodgy chemicals (you can even pre-cut the strips so no risk of scissor injuries!)

thanks Ann great idea!

As it's getting autum you could do this with leaves, to explain why leaves change colour in autum. Here's a useful link, although it's essencially what Anne said:

http://www.sciencemadesimple.com/leaves.html#autumn_leaves_science_project

I was also asked to do this project two years ago and decided ageainst it. I didn't feel that it is a good idea to tell people to go into science.

Actually

http://www.sciencemadesimple.com/

have some really good experiments to choose from.

Get some lungs from the butcher and inflate them with a straw. Just remember to remove your mouth from the straw before they deflate or you get a gobful of minging lung-air.

Have you ever made exploding custard or Alka Seltzer rockets?
http://www.stevespanglerscience.com/experiment/00000068

I was involved with techfest in Aberdeen one year, maybe have a look at their website for ideas:
http://www.techfest.org.uk/index.php

Papasean, great ideas but don't forget that we are living in 'nanny state' Britain and unfortunately, it's all about COSHH and risk assessments:

Bird droppings = biological hazard (Cryptococcus neoformans, histoplasmosis, Salmonella, E.coli etc)
Apple pips = contain amygdalin, a compound that is metabolised to form hydrogen cyanide.
Conkers = Children should apparently wear eye protection whilst handling these [see http://news.bbc.co.uk/1/hi/england/cumbria/3712764.stm]

It's no wonder children are losing interest in science these days, they're not allowed to do anything fun!