Amazon's  (AMZN)  annual Prime Day sales event has already drawn in millions of shoppers since it started at 9 p.m. ET on Monday, the company said. 

The best-selling item so far in the U.S. is the Amazon Echo. The smart home device powered by Amazon's voice assistant Alexa is on sale for $89.99, 50% off its regular price of $179.99. Other best-sellers so far include the Echo Dot, on sale for $34.99 vs. its regular $49.99 price, and the new Fire 7 Tablet with Alexa capabilities, which is selling for $29.99 vs its regular $49.99 price tag. 

Last year, Amazon's top-selling device globally on Prime Day was the Amazon Fire TV Stick that includes a voice remote with Amazon's Alexa voice assistant built into it. Alexa lets you pick movies or check the weather with simple voice commands. The top-selling item in the U.S. last year on Prime Day was an Instant Pot 7-in-1 multi-functional pressure cooker that is supposed to make cooking restaurant-quality meals easy even for cooks that typically rely on ramen packets. 

This year, the Amazon Echo is the best-selling item of Prime Day so far, Amazon said. 

Amazon's best deals tend to be on its own devices like the Echo and the Fire TV Stick, since Amazon has a lot to gain from surrounding its customers and embedding its products into people's everyday lives, according to Edward Jones tech analyst Josh Olson. "Amazon's ecosystem is becoming so broad as it draws people in with its services to create a stickier customer base and sort of lock customers in," he explained. 

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The Amazon Echo device was in 12.3% of U.S. households in the 2017 second quarter, according to estimates from Cowen and Company that were released in a note Tuesday morning. In addition, 85% of Echo owners used the device at least once per day in the 2017 second quarter, up from 81% in the first quarter of 2017. 

Amazon is expected to do between $800 million and $1 billion in sales on Prime Day 2017 which ends at 3 a.m. ET on July 12. Amazon's first Prime Day in 2015 is estimated to have made about $400 million in sales and Prime Day 2016 sales estimates are between $500 million and $700 million. 

Amazon's stock was trading slightly lower on Tuesday morning to $989.45 after climbing about 1.8% on Monday. 

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