Can't be gravitational, due to the fact that atoms are way too small to have a gravitational pull for something of similar size.
The strong nuclear force is the answer you're looking for.
Inhaling through the nose
Despite their frequency in the intertidal zone, in tide pools, sea urchins can be found at many different depths and in any habitats. They can also be found in nearly any ocean temperature. Sea urchins inhabit the polar seas as well as the warm tropics.
Primary reinforcers have distinctive reinforcing qualities; not learned. In addition, secondary reinforcers has no inherent value and only has reinforcing qualities when linked with a primary reinforcers. The positive reinforcement is a desirable stimulus is added to increase behavior while negative reinforcement is an undesirable stimulus is removed to increase behavior.