This small solenoid valve lets you connect a 3mm tube to either end.
The valve is closed by default. When 3V is applied to the Control line on the USI the valve will open and allow a liquid or air to flow through for as long as it's open. 
 
Potential uses include:
Automatic watering system: Control a small water line to drip-feed a plant in a science project.
Mini water fountain: Switch the flow on/off to make a simple fountain or squirter.
Liquid experiments: Measure flow rates by timing how long it takes to fill a container when the valve is opened.
Closed-loop control: Combine with another sensor to stop flow automatically when a condition is met.
Chemistry: control the delivery of chemicals in a chemistry experiment
 
The valve comes with two 20cm lengths of silicone tube, the valve USI and some extension jumpers.
 
NB. Not suitable for hot or hazardous materials.