This can create thermal shock which causes the glass to expand and crack under the pressure. However, motoring experts have urged caution over the simple trick, claiming there was still a big risk of vehicle damage. Neil Greig, IAM Road Safety expert warned the melted water could “refreeze” in windscreens and even risk “icing up locks”. He said: “Anything involving applying warm water to a frozen windscreen does risk causing a crack or making an existing one even worse. READ MORE "The melted water may refreeze on wipers or in the engine compartment and using it on a side window also risks icing up locks or door handles.