Can you explain the cost and elasticity in economics in a popular way? Rich or lacking? Thank you.

There are many kinds of costs. I guess you mean opportunity cost. The opportunity cost in economics is the biggest profit you can get when you use a resource for a certain purpose and treat it for other purposes. For example, if you go to graduate school for three years, then the opportunity cost during this period is the biggest benefit you can get from doing other things in these three years.

Elasticity refers to the sensitivity of one variable to the change of another variable. Elasticity means sensitivity, and lack of elasticity means insensitivity. For example: Apple's sales volume is one variable, and Apple's price is another variable. If the price of apple changes greatly, but the sales volume changes little, it is inelastic; If the sales volume changes greatly, it is flexible.

Of course, the above explanation is only rough and not rigorous.