Find a jar with an airtight lid. Punch a hole in the center of the lid with a hammer and nail. Thread a straw through the hole. Seal the edge around the hole and straw with clay. Draw a smiley face on the marshmallow. Place it in the jar and tighten the lid. Show your child how to use your mouth to slowly suck the air out of the jar. Watch the marshmallow screw up into a caricature of its former self, and the happy face distort into something funny. Marshmallows are full of air and as you suck the air out, the air is sucked out of the marshmallow, too. Discuss with your child why this happens.
Teach your child what happens to a marshmallow when you microwave it. Put a 'mallow on a paper plate and put it in the microwave on high for one minute. Let your child watch it cook. As the marshmallow heats up, it will start to expand to quadruple its size. Remove the marshmallow and examine it with your child. Allow him to manipulate it into a different shape. It will stay that way and become crunchy. Discuss with your child what happened to the sugar of the marshmallow when it cooked. The microwave process heated up the water in the marshmallow, which warmed the sugar and the air bubbles. The gas molecules started moving very fast and pushed against the walls of the marshmallow, causing it to expand. When you removed the 'mallow from the heat, it cooled and the sugar hardened, causing it to become crunchy.
Make three marshmallow recipes and try out different ratios of corn syrup to sugar. Label three greased aluminum cake pans with numbers 1, 2 and 3, to keep track of the recipes. Dust a little powdered sugar inside each pan. Mix together 1/6 cup cold water and a packet of unflavored gelatin in a large mixing bowl. Set the bowl aside. Next, add 1/4 cup water to a saucepan. Add granulated sugar and corn syrup to the water according to each recipe. The ratio of sugar and corn syrup can be: 1/2 cup sugar to 1/3 cup syrup; 2/3 cup sugar to 1/4 cup syrup; and 3/4 cup sugar to 1/6 cup syrup. Put the lid on the pan and turn the heat to medium-high. Check the mixture every 30 seconds until it starts to boil. Then take off the lid and check the syrup temperature with a candy thermometer. When the temperature reaches 240 degrees Fahrenheit, remove the mixture from the heat. Pour it into the gelatin and water, and turn the mixer to a low setting. Set a timer for 11 minutes and slowly increase the speed of the mixer until it's on high. When the syrup starts to become thick, pour in 1/2 tsp. of vanilla and mix it for another minute. Spoon the mixture into the cake pan with an oiled spatula. Have your student write the entire process down in a log book, and start again with the next recipe. Later, cut out marshmallows with cookie cutters, sprinkle them with powdered sugar, and seal them in plastic bags. Perform taste tests, cutting tests or melt tests as part of your science experiment.
Make a model showing the Earth's layers with marshmallows and other candy for an Earth science project. Make a hole in the middle of one large marshmallow and put in a maraschino cherry as the earth's core. To make the mantle, melt butter and mix in about 12 more marshmallows. Remove from heat after they're hot, and add two cups of crispy rice cereal. Allow it to cool for five minutes. Roll the mixture into a ball around the core. Make the Earth's crust by spreading a layer of peanut butter over the mantle. Roll the ball on a plate of crushed-up chocolate cookies. Slice Earth down the middle and you can see the inner and outer core, mantle and crust. Label the layers and display at school in the science fair.