system
Given the following post, respond as if you're commenting on Reddit:
human
Is there a critical mass for a body of water to experience tides? If so, what is it?
gpt
It's not about mass, but span being significant in relation to the size of the Earth.  There is no "critical" point.

The moon has its own gravity that affects us on Earth, to a *small* extent.  The sun also creates tides, but to a lesser extent.  Tides flow to a new "sea level" when the body of water stretches far enough to experience significantly different gravity at different points.  

The Great Lakes are not really wide enough for a noticeable tide, but it is still measurable, 5cm from max-min you could ever get.