any ideas?????we have an exhibition in school i have to do an interesting and fun physic experiment what shoul?

Depends on the time you have, if its a short presentation you could 'demonstrate' the effect of static electricity - I've done this a few times along with other similar demo's at our company technical seminar and it went down well. Very simple.  You need a running cold water tap OR a glass laboratory burette with a tap at the bottom, a lab stand and clamps,  2 small beakers, glass's or cups and a comb. Set up the equipment with one cup underneath the tap and one next to it. Fill the tube with cold water (or run the tap) Now you need the stream of water to be as thin and continuous as possible. The rub the comb through your hair or on a nylon shirt or sheet to generate static on the comb i.e. 'charge' the comb (I actually used a live rabbit, fur was very good - and funny).  Now hold the back of the comb very close to (but not touching) the thin stream of water. With a little practice you will be able to divert the water to one side and fill the cup that's not under the tap.  You can create your own 'patter' or chat to support this, either as a magic trick or whatever. Then finish by explaining the science - which is that the 'ions' in the water are attracted by the static charge in the comb.  It's a bit of a quick explanation but hope this gives you an idea. Practice this first.  Good luck.