preposterous - Contrary to reason or common sense; utterly absurd or ridiculous.
When Sandy's stock broker called and suggested that he sell half his stocks because he felt the market was overvalued, Sandy thought the idea was preposterous - but it turned out that his stock broker was right.
tinker - To attempt to repair or improve something in a casual way, often to no useful effect
Mitch tried to tinker with his computer before realizing that it was more broken that he had thought, and that he had to bring it to the computer store to have a professional repair it.