The "Burn a Peanut Lab" is a well-known approach to measuring calories. The peanut is shelled, skinned, and skewered on a paperclip. It is then burned under a measured mass of water, for example, 200g. A thin metal container, such as a soda can, works fairly well. The temperature is measured both before and after and from these data you can determine the calorie content.

Calories = (Mass) x (∆T)

Here, the mass is in grams, and the ∆T, or change in temperature, is Celsius. The surprise is that there are usually thousands of calories in the peanut, which makes no sense. The lesson is that there are two types of calories: heat calories and food calories. Food calories are 1000 heat calories, also known as a kilocalorie. So, your 2000 calorie diet is really a 2,000,000 calorie diet!

A safer experiment, or a follow-up experiment, is to measure the calories in a heat pack. You do not necessarily have to tell the students the instructions. They can try to figure out a process on their own. The way I usually do this experiment is to place one in an insulating container, with about 200g of water, and click the button. The water will begin to warm up. Don't forget that 1g is 1mL for water.

Clicking the button forms a tiny crystal seed, called a nucleation site. From this seed, the other crystals grow. The crystal seed is necessary because the liquid in the heat pack is chemically pure. In the case of snowflakes, the seed is usually a speck of dust, but this pack contains pure Sodium Acetate which is super-saturated in water. That means that there is so much sodium acetate that cannot stay dissolved in the water and should be solid (like too much sugar at the bottom of a Kool-Aid mix). Sodium acetate is non-toxic and is even added to food as a seasoning. Chemically, it is a vinegar salt. Perhaps you have eaten it on potato chips?