Given a scientific question, generate a correct answer to it.

Input: Consider Input: Radioisotopes may vary greatly in their rate of decay, decaying faster if their nuclei are more what?

Output: unstable

Input: Consider Input: How long do the long-lived isotopes require to decay to a safe level in a nuclear reactor?

Output: thousands of years

Input: Consider Input: What forces in liquids are strong enough to keep them from expanding significantly when heated?
Output: intermolecular forces