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We consider query slipping and its prevention in wireless sensor networks. The problem happens when a query propagating along a subscribe trajectory moves through a publish trajectory without obtaining the desired information, even though the two trajectories intersect geometrically. There follows resubmission of the query or initiation of another subscribe trajectory. Thus, query slipping results in considerable time delay and, in the worst case, looping in the trajectory or query flooding the network. We address the problem formally and suggest a solution. First, the area where nodes are distributed is logically partitioned into smaller grids, and a grid-based next-hop selection algorithm is proposed. The algorithm not only attempts to make the trajectory straight but also considers the nodal density of recipient nodes and the seamless grid-by-grid multicast. We show that publishing and subscribing using the algorithm eventually eliminates the possibility of slipping. It turns out that our approach dissipates less power at neighbor nodes, compared to a non grid-based method, such as greedy forwarding.