Look at this series: 2, 1, 4, 2, 18....... What number should come next?

This series appears to be alternating between two different sequences.

The first sequence is: 2, 4, which seems to be multiplying by 2. The second sequence is: 1, 2, which seems to be multiplying by 2 as well.

If we continue these patterns, the next number in the first sequence would be 2 * 2 = 4, and the next number in the second sequence would be 2 * 2 = 4.

However, the series ends with a number from the first sequence (18), so the next number should come from the second sequence.

Therefore, the next number should be 4.