We have seen lots of corn starch experiments in the past. When mixed with water it is a liquid that can be poured but if you shock it with a hard impact it will turn into a solid. There are lots of fun experiments that can be done with it but the TheBackyardScientist has recorded some great reactions with his high speed camera. The best reaction I think is the one at 2:50 where a golf ball is shot at a balloon filled with the liquid, in real time it looks like a normal explosion but in slow motion you can see that the substance breaks apart like chunks of fruit.