Only 3.125% of the Earth's total surface is available for growing crops, which is represented by a tiny sliver of an apple. The document compares the Earth to an apple, with 3/4 being covered by water and 1/4 being land, half of which is unsuitable for crops due to conditions like being swampy, desert, or mountainous. A further 3/4 of the remaining land is also unusable for crops due to being too developed, rocky, or wet/hot, leaving only 3.125% available for crop production.