When you take a shower, you turn the water on, wait for it to get hot, then you get in.
It's that second step that's the problem, environmentally speaking: As a company called Evolve Showerheads points out, while you're waiting for the agua to heat up, "two-and a half gallons of hot water per minute are funneling down the drain."
Evolve designs the solution into their products:
Utilizing the unique, patent-pending ShowerStart technology, [our] showerheads [stop] the flow to a trickle when the water reaches a comfortable 95 degrees. This way your hot water's no longer running down the drain while you're away from the shower.
Using an evolve showerhead can save a yearly average of 2,700 gallons of water, the fossil-fueled energy it takes to heat it, and up to $75 off your utility bill.