Energy Density
Energy density is how much energy a fuel holds for its weight or size.
Think of it this way
A jar of peanut butter and a jar of water sit side by side. Each jar is the same size. The peanut butter jar holds far more energy than the water jar. That is energy density — more energy packed into the same space.
Explaining energy density by grade level
Some fuels hold more heat than others. When you burn a small drop of gas, it warms water a lot. A drop of plant fuel warms it less. The fuel that heats water more has more stored energy.