This is a great question that gets to the heart of how genetics works! The idea that DNA is only composed of three chemicals is incorrect. DNA is actually made up of four different chemical building blocks, called nucleotides. These nucleotides are:

* Adenine (A)

* Thymine (T)

* Guanine (G)

* Cytosine (C)

These four nucleotides are arranged in a specific order within a DNA molecule, forming a kind of code. This code is what determines the instructions for building and maintaining an organism.

Here's why differences in living things arise even with just four nucleotides:

* Sequence: The order of these four nucleotides is what matters. Imagine a sentence like "The cat sat on the mat." Rearranging the letters changes the meaning entirely. Similarly, rearranging the order of nucleotides in a DNA sequence can lead to different instructions, resulting in different traits.

* Length: Different genes (sections of DNA that code for specific traits) can be different lengths. A longer gene might have more nucleotides, allowing for more complex instructions.

* Combinations: The four nucleotides can be combined in countless ways. The human genome, for example, has about 3 billion base pairs (combinations of A, T, G, and C). This vast number of combinations allows for incredible diversity.

* Regulation: How genes are expressed (turned on or off) is also important. Different environmental factors, internal signals, and even interactions between genes can influence which genes are active at any given time, further contributing to variation.

Think of it like this:

* The alphabet: You only need 26 letters to create countless words and sentences.

* The DNA code: You only need four nucleotides, but their sequence, length, and regulation create incredible diversity in living things.

This is why even with just four building blocks, DNA is incredibly complex and capable of producing the vast array of life forms we see on Earth.