Here are more examples from the radio programme:

$1$ I saw a pale student kneeling beside the monster he had put together.

$2$ There was a red screen formed by flowing blood.

$3$ I could see long rows of atoms twisting like a snake.

$4$ Benzene molecules, like the snake made of atoms, have the shape of a ring.

$1$ Which clauses in $1-4$ are made with:

a present participles $(-$ing$)?$

b past participles $($often$-$ed$)?$