How much does that sweater cost in Bitcoin? Depends. Bitcoin (and all other cryptocurrencies) are volatile, with values changing minute-to-minute, day-to-day. That's one reason retailers might be hesitant to adopt them as payment.

Sam Cox, a technologist at Rehabstudio London, is hoping to change that with BitTag, a physical price tag that updates itself automatically. The hardware shows the name of the product, price in real currency, and the real-time cost in Bitcoin. The price updates as soon as the value of the cryptocurrency changes -- so customers, and retailers, aren't caught off guard.

The tag is managed using an iPad app, and if it's shaken, will enable a QR code to activate which can be scanned, which would complete the transaction.