When encountering liquid food sources, ants of the genus Aphaenogaster drop various materials as tools into the food, and then carry the food-soaked tools back to the nest. Although this is one of the most well-documented examples of tool use in insects, we know little about which factors influence their choice of tools during foraging. Here, we investigated the tool-using behavior of Aphaenogaster subterranea by examining, across a range of settings, how tool-using workers deal with various foraging challenges. We used different types of tools and liquid baits with varying distances between the baits and the tools piled around them. Although ants showed the strongest preference for the most easily transportable tools, the frequency of usage of the less preferred larger tools increased when these were the first to be discovered or were more readily accessible to the ants. Additionally, tool selection occurred at two stages: when tools were picked up and dropped into baits, and later, when food-soaked tools were retrieved from baits. Our results confirm that tool use in A. subterranea exhibits a high degree of flexibility. Ants seem to be able not only to optimize their foraging effort by selecting tools that are best matched to the particular foraging environment, but also to learn how to improve the use of certain tools by modifying them. The adaptive value of flexibility in tool use may be the increased efficiency in the utilization of liquid food sources, which gains importance in the light of competition with co-occurring ant species.