28.5. Dab tsi yog cov ua rau ntug kev tawg?

Thaum lub sij hawm vuam txheej txheem, yog tias ib qho kev nyuaj siab ntawm ntug kev, ua rau muaj kev nyuaj siab ntawm Weld, ua rau muaj kev kub ntxhov ntawm ntug kev kub ntxhov. Muaj ntau cov laj thawj uas tuaj yeem ua rau ntug hlawv - los ntawm qhov tsis xws luag. Piv txwv li, yog tias lub vuam ceev nrawm nrawm lossis lub siab vuam tam sim no yog siv thaum ua cov txheej txheem, nws tuaj yeem txhim kho cov arc thiab ua rau muaj qhov tsis xws luag. Ib qho ntxiv, cov teeb meem nrog cov vuam tshuab weld pas dej tuaj yeem ua rau muaj kev hloov pauv hauv vuam qhov tob thaum lub sij hawm. Nyob rau hauv cov ntaub ntawv no, nws nyuaj rau weld hlau los sau kom zoo, coj mus rau qhov tseem ceeb phenomena. Yog li ntawd, thaum ua yeeb yam muab cov roj hlau vuam, nws yog qhov tseem ceeb kom tsis txhob muaj qhov tshwm sim ntawm ntug ove}}} los ntawm tsis xws li ntau tau.