This phenomenon occurs cyclically.

You need to use your head to solve this problem.

Please read this instruction manual carefully before using the product.

I expressed my gratitude to the teachers.

The use of mobile phones is prohibited during class.

There is a class observation at my son's school tomorrow.

An isotope refers to elements with the same atomic number but different numbers of neutrons.

Rare earth elements are essential materials for the manufacture of electronic devices.

Transition elements play an important role in chemical reactions.

As if symbolizing the heat of summer, the cicadas started to sing.