During the lottery, Portia orders her band to play music while Bassanio determines which casket he's going to choose. The song that's performed offers a subtle hint about which casket is the right choice:
Tell me where is fancy bred,
Or in the heart, or in the head?
How begot, how nourished?
It is engender'd in the eyes,
With gazing fed; and fancy dies
In the cradle where it lies. (3.2.1)
These lyrics warns that "fancy" (love) can't grow if it's "engender'd in the eyes" (based on mere physical attraction) instead of the heart. So we're not surprised when, after listening to this tune, Bassanio declares that "outward shows" least express the truth. This is just before he chooses the leaden casket, which, as we discuss under "Gold, Silver, and Lead Caskets," is a symbol of inner beauty and modesty.