The half-life of Uranium-238 (U-238) is a constant, approximately 4.5 billion years, and does not change regardless of the number of U-238 particles present. Increasing the number of U-238 particles in a sample does not affect its half-life; it simply means that there are more particles that will decay over time.
What this teaches us about half-life is that it is an intrinsic property of a radioactive isotope and is not influenced by the quantity of the substance. It remains constant under the same conditions, making it a reliable measure for understanding the decay of radioactive materials. Therefore, the correct response is: "Half-life is a constant number."