You find a box in the attic.
In this box is a bag of seeds labeled “Iron Tree”. When planted, the seeds will grow into what appear to be oak trees. The rate of growth is normal for oak trees given proper watering and sunlight. Examining the acorns will reveal, beneath the cap and outer skin, an ingot of pure iron.
Also in this box are bags labeled “Mercury Trees”, “Gold Trees”, and “Weapons-Grade Uranium Trees”.