Boat. Rock. Lake. One Question That Stumps Even IITians in IIM Interviews

IIM Interview Question - “You are in a boat with a rock. You throw the rock into the lake. Does the water level go up, down, or stay the same?”

Step 1: While the Rock is in the Boat
While the rock is in the boat, it adds mass to the boat. This mass causes the boat to displace more water, and as a result, to be lower in the water.

The volume of water displaced is equal to the weight of the rock (thanks to Archimedes' principle).

Step 2: When You Toss the Rock Over the Side
The rock sinks. But here's the catch: now in the water, the rock only displaces its own volume of water, not its weight.

Scaling rocks have a greater density than water - that means:

When floating in the boat, it displaces more water (to weight).

When sunk in the lake, it displaces less water (to volume).

Answer: The Level of the Water Goes Down!

Why? 

Because when the rock was in the boat, it displaced more water (to weight) than it does while it is in the lake (to volume).

So, the total water displacement decreases, and the water level goes down.

This question is not about physics formulas – it is about:

• Using basic logic in familiar situations.
• Using decisive reasoning to challenge your intuition.
• Clearly expressing a complicated belief within time limitations.

In an IIM interview, success is not simply about getting to an answer, but about how you think on your feet.