A simple, low-cost and reliable method for the formation of double emulsions is proposed and demonstrated experimentally. The formation process consists of two steps: (1) the formation of water-in-oil droplets at a co-flowing structure formed by a syringe needle and a flexible microtubing, and (2) the formation of water-in-oil-in-water compound droplets at the tip of the microtubing by buoyancy. Since the droplets flow directly from the first step to the second step in the tubing without any disturbance, problems such as droplet coalescence, breakup, leakage and contamination can be avoided automatically. The breakup processes of the core droplets and the compound droplets are analyzed. The sizes of the core and the shell droplets and the volume fraction in the compound droplets are controlled by adjusting the corresponding flow rates. Scaling relationships for the number of cores and the size of the compound droplets are proposed. Due to the simplicity of this method and the flexibility in controlling the properties of the double emulsions, this method shows great potential in the relevant applications.