Students have difficulty understanding synaptic integration in neural circuits, and how spatial and temporal summation combine in a target neuron to reach threshold. This Lesson uses small cups of water to represent excitatory postsynaptic potentials (EPSPs). Cups of water/EPSPs are added to a large cup, using either many cups at the same time to represent spatial summation, or one cup refilled repeatedly to represent temporal summation; students try to fill the cup to a level (line) that represents threshold for an action potential at the axon hillock. This large cup has a hole in the bottom representing the passive leakage of potential as the EPSPs spread from synapses on the dendrites and cell body to the axon hillock of the target neuron. To get the most out of this Lesson, students need to understand the parts and functions of a typical neuron, the basic sequence of events that occur at a chemical synapse, and the concept and importance of action potentials and their frequency. Students also need to understand the difference between a local potential and an action potential. Students work in groups of four to complete a series of activities in which the size of the cup (amplitude of the EPSPs), and the distance to the axon hillock were varied. We analyzed student responses to questions about the activity and found that the Lesson helped them understand spatial and temporal summation and raised some insightful questions about the process of synaptic integration.