Recent studies on CTC (cross-Technology communication) have demonstrated the possibility of building direct communication between heterogeneous wireless communication technologies, such as WiFi and ZigBee. However, current solutions suffer from significant spectrum usage or limited throughput. To address the issues, this paper presents Transparent Cross-Technology Communication (TCTC), a novel CTC technique that establishes a side channel by exploiting today's abundant wireless traffic. The key idea of TCTC is to embed messages in the timings of on-going legacy traffic (e.g., HTTP packets in WiFi), with small and bounded impact on upper-layer applications. Feasibility and effectiveness of our design has been validated via test-bed implementations and experiments on bidirectional communication between WiFi and ZigBee platforms.