Dab tsi yog cov khoom ruaj khov lub qhov rooj?

Hollow cov qhov rooj tseem ceeb yog hom qhov rooj ua nrog ib tus core core thiab veneer.

Thaum lub cuab yeej ntoo yog ua rau cov ntoo nkaus xwb, cov khoom siv cov khoom siv hluav taws xob rau sab hauv thiab cov ntoo hluav taws xob ntawm sab nrauv.