Recent advances in Cross-Technology Communication (CTC) have improved efficient cooperation among heterogeneous wireless devices. To date, however, even the most effective CTC systems require these devices to operate in the same ISM band (e.g., 2.4GHz) because of the conventional wisdom that wireless transceivers with different (fundamental) frequencies cannot communicate with one another. Our work, which is called TiFi, challenges this belief by allowing a 2.4GHz WiFi receiver (e.g., a smartphone) to identify UHF RFID tags, which operates at the spectrum between 840 ∼ 920MHz. TiFi does not require changing current smartphones or tags. Instead, it leverages the underlying harmonic backscattering of tags to open a second channel and uses it to communicate with WiFi receivers. We design and implement TiFi with commodity WiFi chipsets (e.g., Broadcom BCM43xx, Murata KM6D280 40, and Qualcomm WCN3990). Our comprehensive evaluation shows that TiFi allows WiFi receivers to identify UHF RFID tags within the range of 2 m and with a median goodput of 95%, which is comparable to today's mobile RFID readers.