To complete a full circle, as I attempted these in my first presentation, I ran the session on tea bag rockets. To try, empty out the tea bag, chop both ends and stand the cylinder upright (ne careful of fire). You set fire to the top, as it burns it heats up the column of air. This increases volume so decreases density (mass is the same) which allows the hot air to rise to the top, in convection currents. This causes an upwards force to launch the tea bag (made easier by the smaller mass as the tea bag is burnt). This was my last individual presentation, which is upsetting after a fantastic year leading Eng soc!