(the fur of) a kind of animal that lives on land and in water and makes dams across rivers

eager beaver

a machine used for changing steam-power, water-power etc into electrical energy

go-getter

an organic chemical substance formed in plants and animals, able to cause chemical changes without being changed itself

something which incites activity

a drink (e.g. coffee, brandy), drug etc that increases bodily or mental activity; anything that encourages a person (e.g. praise, hope of gain)

substance that invigorates