Take care with fire or heat
Do it outdoors
Materials you need are:
a wooden peg
a long piece of thread (about 50 centimetres long)
a couple of marbles
While squeezing the ends of a wooden peg together, tie the long thread tightly around the ends so that it doesn't spring back. Lay the tied peg onto a flat surface. Put the marbles on either side of the ends you have tied. Light a match and burn the thread only. Watch the marbles shoot away.
When the ends of the peg are squeezed together, it has elastic potential energy. When the thread burns and the ends are released, this energy is transformed or converted into kinetic or moving energy, flinging the marbles away.