Solar energy is a form of renewable energy, namely energy that cannot be depleted. Other forms of energy like carbon based sources of energy cannot be renewed in a reasonable span after being used (forests take time to grow and oil is created in geological timescales). Joe is trying to deal with the phenomenon of resource depletion. The way he goes about it is making a renewable source of energy more efficient and usable.
They were veary diifent so tehy make diif genes
The tendency of later learning to hinder the memory of previously learned material.
