We focus on a sequence of 9 lava fountains from Etna that occurred in 2011, separated by intervals of 5 to 10 days. Continuous measurements allowed to discover the occurrence of gravity decreases before the onset of most fountaining episodes. We propose that the gravity changes are due to the pre-fountaining accumulation of a foam layer at shallow levels in the plumbing system of the volcano. Relying on the relationship between amount of gas trapped in the foam and amount of gas emitted during each episode, we develop a conceptual model of the mechanism controlling the passage from Strombolian to lava fountaining activity. Gas leakage from the foam layer during the late stages of its accumulation increases the gas volume fraction at upper levels, thus inducing a decrease of the magma-static pressure in the trapping zone and a further growth of the foam. This feedback mechanism eventually leads to the collapse of the foam layer and to the onset of lava fountaining. The possibility to detect the development of a foam layer at depth and to set quantitative constraints on the amount of trapped gas is important because of the implications for forecasting explosive eruptions and predicting their intensity.