Is lawnmower a tool or asset?

a lawn mower is a tool, an asset is something that adds value , like a house, a boat or a pension plan

Both. Firstly it is is tool to cut the grass, but secondly as it has value it is an asset of the person who owns it. Under the right legal circumstances it can be taken away from the owner to pay (or part-pay) a debt, or it's value could be used in the calculation to find a person's total "wealth".