My second is performed by my first and it is thought a thief by the marks of my whole might be caught. What am I?
Answer
A Footstep.
More What am I Riddles
6 in roman
After teaching his class all about roman numerals (X = 10 IX=9 and so on) the teacher asked his class to draw a single continuous line and turn IX into 6. The only stipulation the teacher made was that the pen could not be lifted from the paper until the line was complete.
Draw an S in front of the IX and it spells SIX. No one said the line had to be straight :)