Harder example sentences

I kept this up for some minutes but it wouldn't budge, so I pulled harder and harder, with more grunts.Then the python coiled himself round Golu's stomach and said, “Let's pull harder.Soluble fluoride is often added to drinking water to bring its concentration upto 1 ppm or 1 mg dm– The F– ions make the enamel on teeth much harder by converting hydroxyapatite, [3(Ca3(PO4) Ca(OH)2], the enamel on the surface of the teeth, into much harder fluorapatite.