Here are six sets of containers. The water in the top one is draining into the bottom one at a constant rate. The puzzle here is to determine, at five intervals during the pour, what the surface of the water looks like when viewed from above.
Here are the possible choices. There is one set for the surface of the upper one and one set for the surface of the lower one. Some of these cards have missing pieces. Students need to draw those.
Print them from the source below.