Heat Shock
Heat Shock is a quick temperature change that opens tiny pores in bacteria so DNA can slip inside.
Think of it this way
A glass jar sits in warm water, then moves to an ice bath. The quick cold snap makes the jar walls shrink a bit, opening tiny gaps. Small bits floating nearby slip through those gaps and into the jar. When the jar warms back up, the gaps close and the bits are trapped inside.
Explaining heat shock by grade level
Bacteria are tiny living things too small to see. Scientists want to put new parts inside them. They warm the bacteria up fast in hot water. The quick heat opens tiny doors in the bacteria so new parts go in.