Chemists are interested in energy from chemical reactions. Most of our heat energy comes from changing chemical energy into heat (eating food or burning fuel). Reactions like this are called exothermic (exo means "out", as in the word "exit"). The opposite reactions, during which heat is taken in, are called endothermic. Set up some endothermic and exothermic changes.
Endothermic are difficult, but dissolving salts such as potassium, or ammonium, nitrate in water will make the container feel colder (this is how the Romans cooled their wine, using saltpetre and water). Mixing lemon juice and bicarbonate of soda, or adding salt to ice will make the temperature fall considerably. Exothermic change is easier: add magnesium powder to acid; or iron filings to copper sulphate. Or try a combustion reaction. Very carefully set a few drops of an alcohol alight, and show how lots of heat comes out in this flame. Chemicals contain energy in their bonds; depending on pupils' ability you might discuss why some chemical changes are exothermic and others endothermic, in terms of the energy in the bonds that are broken compared to that in the new bonds that are made.
Generally, small animals lose heat more easily than larger ones, hence the elephant has big ears to increase its surface area so to help it keep cool, and yet the mouse is quite furry to keep its heat in. This is explained by the ratio of surface area to volume, and also tells why babies need to be kept especially warm, as they have a relatively small ratio of volume to surface area.
For an investigation, start with hot water in a small and a large container at the same temperature. Place a thermometer in each and plot a graph of temperature against time over 10 minutes or so. Which keeps the heat in best? You might compare other shapes. Very able students can work out the sizes and do the sums.
This can be further adapted to show which materials are the best insulators, by wrapping them around the small container to see how well they slow the heat loss; try plastics (eg polystyrene), fur or wool (maybe also different thicknesses); better still try the coat sleeves of pupils'
outdoor coats. See who stays warm!