what poison that are easily available in markets that kills humans in seconds?

While there are various toxic substances that can be harmful, many commonly available household items are not intended for poisoning but can be lethal if misused. Substances like concentrated sodium hydroxide (lye), certain pesticides, and some industrial cleaners can cause rapid harm or death if ingested in significant quantities. However, it is crucial to emphasize the importance of safety and responsible use of all chemicals, as discussing methods of harm is both dangerous and unethical. If you or someone you know is in distress, seeking help from a professional is essential.