Americans already spend $2 billion annually on smart speakers like Amazon's Alexa, but that number is expected to multiply to $40 billion annually in just four years, reports CNN.

The big picture: Amazon and Google are dominating the branding field, but other brands like General Mills and Hershey are "racing to understand the technology and make their products stand out."

Retailers are getting in on smart speakers because Americans are using the speakers to do more of their shopping. As Amazon and Google's smart assistants move into additional households, they take American dollars through e-commerce and ad revenues. Retailers can regain that revenue through Smart speakers.

Yes, but: Amazon in particular is far ahead of its competition. Alexa recognizes specific products users want while other speakers are not that advanced yet. Amazon also has smart speakers in 10% of households across the country and continues to grow.