Gil found a nearly new engine in his neighbor’s garage. He was experienced enough to see that it was in good shape. When he asked the neighbor how
much she wanted for it, she said “you can have it if you’ll get it out of here by
the weekend and clean up the oil and grease around where it was”. Gil said,
“Okay”, even though he knew the engine was worth nearly $1200. Is this a
valid contract?

There are two ways to view intelligence or ability:
ability it is fixed or ingrained – in other words, we are born with a certain level of ability and we cannot change that. this is called a fixed mindset.
we can develop our ability through hard work and effort. this is called a growth mindset.
these two different beliefs lead to different behaviour, and also to different results. for example, students with a growth mindset were shown to increase their grades over time. those who believed that their intelligence was ingrained did not; in fact, their grades got worse.
having a growth mindset (the belief that you are in control of your own ability, and can learn and improve) is the key to success.
yes, hard work, effort, and persistence are all important, but not as important as having that underlying belief that you are in control of your own destiny.