What is the solution to this logic puzzle?

Through some unexplained means, a rock has become sentient (conscious). However, to remain sentient, the rock neads to get energy (of any type). Nearby, there is a squirrel and a mushroom. There is lichen growing on the rock, and it is sunny out. How does the rock get energy? Anything that I did not specify about the objects can be assumed in the answer (The mushroom could be poisonous, or not.) Try not to make unresonable references to things not in the question (you should not refer to a house, but you can refer to soil). I shall deem the person who writes the most complicated answer the “winner”.

As an example of a short answer, you could say that the rock creates a symbiotic relationship with the lichen and gets energy from them.