Using a laboratory experiment, we study the incentives of individuals to contribute to a public good that is provided if and only if the fraction of contributors reaches a certain threshold. We jointly vary the size of the group, the cost of contributing, the required threshold, and the framing of contributions (giving to the common pool, or not taking from the common pool). We find that a higher threshold makes individuals more likely to contribute. The effect is strong enough that in a small group, raising the required threshold increases the probability that the public good is provided. In larger groups, however, the effect disappears. At the same time, we do not find a consistent effect of framing on the probability of contributing or on the likelihood of success.