If you're not careful, most of what you do online and in mobile apps will wind up fueling targeted advertising, all that data feeding into a composite profile of your likes, dislikes, and demographic information.
But increasingly, even careful web users can't avoid being swept up in the digital marketing dragnet.
This means that with just a few taps on either platform, you can disrupt the profiles ad networks have collected about you, and keep them from growing any more, by turning on a feature in both Android and iOS that essentially sends out a dummy ID that's all zeros.
To do it on Android, go to Settings > Privacy > Advanced > Ads and toggle on Opt out of Ads Personalization.
If you don't want to stop ad tracking altogether—you're getting ads anyway, might as well be relevant—you can navigate to those same screens and tap Reset advertising ID on Android or Reset Advertising Identifier on iOS to cycle your ad ID and essentially force advertisers to start a new profile on you.
Android actually shows you your (very long) alpha-numeric ad ID at the bottom of this screen and when you initiate a reset you can watch it change.