I'm not sure what exactly you are talking about so my answer may be wrong. From what I understand MC will first decrease till one point and then start to increase later. So, at first, if you hired a cook for example he would improve the production by producing 20 more pizzas, but as you hired more and more cooks they wouldn't improve the production as much, after a point another cook would only increaase the production a little because the room would be limited and so they wouldn't be able to work very well, so those few extra pizzas that were produced because of that new cook you added would have a higher marginal cost, since you would pay the cook 1k for example but only 4 new pizzas would be added to the production. The supply curve shows the quantity that a company is willing to produce at a given price in order to maximize it's profit. If a pizzeria sold 8 euros for each pizza then it could produce so many pizzas so that the last pizza has a marginal cost of 8 euros. Having a marginal cost of more than 8 euros wouldn't be good for the company, since the creation of the pizza would cost more than it would be sold and thus it would not be profitable for the company to produce it. When we create the supply curve we do not use prices that are below the average variable cost, the reason is simple, if the price of the pizza was less than the average cost that each pizza needs to be made then we would not have a profit, since we would sell the pizzas less than they cost us to make them. So, in the supply curve we use the MC that's over AVC, which is why you see it only increase. If however you are talking about how MC increases when the price for the tomatoes for those pizzas increased, then that's also logical, each pizza would cost a little more, so the marginal cost for each pizza would increase. If however the technology on the other hand improved, then those pizzas could cost less money to make, which would decrease the marginal cost.