What's the joke about Jason Nana selling flashlights in Kaxiu?

It's a line from the movie "Domestic Lingling Lacquer" starring Stephen Chow. There is a dialogue between Da Wenxi in Luo Jiaying and Lingling in Stephen Chow.

The original dialogue is as follows:

Leonardo da vinci: I have recently invented something that I believe can help you.

(Pull out the flashlight, the flashlight is shining)

Lingling Paint: A flashlight?

Da Vinci: Wrong! This is not an ordinary flashlight, this is a solar flashlight without batteries! When there is light, it will shine!

Siling: What if there is no light? ......

Da Vinci: Absolutely not!

Lingling lacquer: Is it possible that it will shine when there is no light?

Da Vinci: Good question! Turn off the lights.

The light is off, it's dark, and the flashlight doesn't work. )

Da Vinci: Where ~ ~ If you take out a flashlight and shine it, it will light up. ......

Shine another flashlight on the solar flashlight, and the solar flashlight will glow immediately.

Hey ~ ~ What's up?

Siling: Turn on the light!

Lingling lacquer: This invention is really creative!

Siling: Hmm. ...