a stopwatch is used to measure time & how long something takes to happen is a kind of measurement of time ->  a stopwatch can be used to measure how long something takes to happen  a stopwatch can be used to measure how long something takes to happen & a process takes time to happen ->  a stopwatch can be used to measure how long a process takes to happen  a stopwatch can be used to measure how long a process takes to happen & melting is a kind of process -> a stopwatch can be used to measure how long it takes for melting to happen