A black dog is sleeping in the middle of a black road that has no streetlights and there is no moon. A car coming down the road with its lights off steers around the dog. How did the driver know the dog was there?
54325. Rather than being a mathematical solution, the answer is derived by prepending (putting in front of) the value on the left side of the equation with the previous answer. So for 3 you take 25, the previous answer, and put 3 in front, giving you 325. Then you put 4 in front to get 4325 and lastly, put 5 in front to get 54325.
A seed. One seed turns into a plant or tree and seemingly disappears. When a plant dies, the seeds can be harvested to create new plants. From a single seed, hundreds more are created, vastly exceeding the original. Gardeners care for plants on bent knee and each new plant helps make the air fresher.
The most popular answer for this teaser is ‘today’. Of course this only works if you’re reading it on that particular date or if the date is not specified. Another option is to respond literally with ‘a date’. Lastly, certain dates could be squeezed into letters using Roman numerals if you abbreviate the year to 2 numerals. For example, January 20th, 2011 would be I XX XI.