The answer to the riddle "If you throw me from the window, I will leave a grieving wife. Bring me back but in the door, and you'll see someone giving life!" is the letter "N".
Remove the letter "N" from the word window leaves you with the word widow. Add the letter "N" to the word door and you're left with the word donor.
The National Institute of Environmental Health Sciences has some "not so hard riddles " to solve.