You will hear a sharp click as electrons move from one wire to the other.
If the earphone came with a jack on the end instead of two bare wires, you will need a piece of metal such as a spoon to connect the two metal parts of the jack.
For now, we will just say that a sensitive earphone has a very high impedance, which is measured in ohms.
A speaker has a low impedance, usually about 8 ohms.
Some crystals such as quartz, and Rochelle's Salt are piezoelectric.
Some ceramics (such as those made with barium titanate) are also piezoelectric.
The most difficult part of building a crystal radio is building an efficient earphone that can convert the tiny electrical signals into tiny sounds that our ears can hear.
Our first radio used a telephone handset for an earphone, and that works quite well.
Those are meant to be driven by a signal loud enough to drive a speaker, and are not sensitive at all.
We will talk later (in the scientific part of this chapter) about impedance and what it means.