Does your mind ever wander off on strange tangents, and you start forming weird and wonderful theories and inventing odd things? Of course you do!

Please share your interesting thoughts, theories and tricks you've figured out. You might not understand what I mean so here are a few examples from my own brain ramblings.
Third Spotting 
Add the digits of any number together, repeat the process on the result until it is a single digit number. If the final result is 3, 6 or 9 the original number is divisible by 3.
99 Fingers
Did you know you can count up to 99 just using your fingers?
Both hands can represent 0-9 independently: zero is obviously your closed hand and the first five digits are counted in the usual way by raising your digits starting at your thumb and working across the hand to your little finger. To represent 6-9 fold down your digits in the same order, working from thumb to little finger (which sets you back to zero). Now using one hand to represent units and the other tens, you can count up to 99!