'To set the Thames on fire' means 'to achieve something impossible'. The Thames is a river flowing along London and Surrey in England. So, the idea of setting fire to the water of a river is like doing something which cannot easily or usually happen.
London repeatedly caught fire in the bygone centuries. The tone of this idiom perhaps sadistically recalls that fact.
Sample Usage
1. Qualifying Civil Services examination for you is like setting the Thames on fire.
2. Sachin Tendulkar set the Thames on fire yesterday by scoring a double century.
3. Ramesh scored cent percent in Maths, earlier he thought it was like setting the Thames on fire.
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