So, for example, we have this average level, and then it can go high where we have the peak, or it can go very low. We can also discuss sound waves, so again it's just the periodic variation of some property -- in this case we're talking about density, so we have high density areas and low density areas.

例如这是平均位置，在峰的位置水位高，在谷水位低，我们也可以,讨论声波它也是某种,量的周期变化，在这里我们,讨论的是密度,我们有高密度区和低密度区。

麻省理工公开课 - 化学原理课程节选

So the fact is those lab animals have a hard-wired preference for foods that are high in fat and high in sugar, primarily because of the energy density that those foods bring to one, and then the variety is important because you get a mix of nutrients.

事实上 实验动物对高脂肪高糖的食物,有着强烈的偏爱,而这种爱基于,该类食物能给我们提供高能量,多样性很重要,因为你能摄取多种营养

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选