Fishig buddy of mine and I have tested this extensively with some wild results. We are often out of cell range while fishing and sometimes we will even power down our phones. We will pick something random and stupid that has nothing to do with us to spend a few minutes talking about, just sitting on the lake with our phones off and in pocket. After returning to service we can turn our phones on and scroll through the Google News feed and every time, without fail an ad will pop up with whatever stupid thing we discussed. Last weeks conversation came out with hilarious results and buddys phone being spammed with sex toy ads for three days following.
On another outing we left both phones sitting on my kitchen table, low and behold our conversation while traveling in his truck was where the ads were targeted upon return.
There is almost nothing you can do anymore without being watched.
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