Directed ads show up of things I have only spoken about

My iPhone or some app must be listening to my conversations and pulling out the subject of the conversation and putting that subject in ads on my web pages etc I'm reading. I have had this happen a few times in the last few weeks. I'll have a conversation with someone, they mention something which I know I have never searched for and the next time I go to a site that has ads, there is the subject of the conversation. The last example is apple cider vinegar. How is this happening, is it legal, please explain?!