A Professor at Ohio University in the US has gone viral after a very touching answer to a student's question.

Assistant professor Dan West teaches a class called Introduction to Human Communication, which includes a section on love and relationships.

In a recent session, West opened the floor to questions from students. He had no idea one question in particular would catapult him to viral internet fame.

A student asked West in class: "How do you know when you're in love?"

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According to Buzzfeed, West's answer started with the obvious: "it just hits you".

"You can't expect it and you can't plan for it," he said.

Then he went on to explain how he knew he was in love with his wife, Vicky, whom he married in 1993.

"We had just come from a movie and stopped by a grocery store so I could buy some food for the next day. We were looking at ice cream and as I watched her choosing I realised that I was going to be buying groceries with her for the rest of my life," he told the class.

He proposed to her shortly afterwards.

A tweet from a student quickly went viral, with more than 77,000 people retweeting the grocery store love story.

via GIPHY