Interactivity is at the heart of blogging culture. Most bloggers do not just hand down their wisdom; they interact with readers. However, some readers are more interested in shouting down or trashing an opinion than civilly disagreeing, and they often do so under the shelter of anonymity. Spammers can quickly overwhelm the comments section with ads for sex sites and unsavory products. All spammers need is a list of web or email addresses, which can either be bought cheaply or "harvested" from the Web by special software. Spammers can send 10,000 emails for less than $100. The increasing sophistication of spam filters has made spammer's jobs harder, but spammers are a determined and knowledgeable lot. Most blogging software now includes a built-in spam filter, which can usually be found on the "Settings" page. Northeastern University's Dan Kennedy finds that the most interesting comments on his blog post now come not from the comments sections however, from social media.