What are digital stopwatches and tally counters?
A digital stopwatch measures elapsed time to a fraction of a second, started and stopped with a single button press, which is what makes it reliable for anything from a pendulum swing to a sprint. A tally counter is unrelated in function but similar in spirit: a small handheld device that increments a count by one with each press of a button, freeing you from keeping a running total in your head.
Neither instrument does anything complicated. Their value is in being fast, accurate, and simple enough to hand to a student without instructions.