A young man wearing goggles is putting some liquid in a beaker while a young girl wearing blue gloves looks down while holding a pen.
A man is lighting a fire under the beaker as part of his demonstration to the girl.
You usually cannot light a fire under a beaker while you are putting liquid in it.
The man is putting some liquid in a beaker or lighting a fire under the beaker, not both.
One does not look down during a demonstration.