Yup. Really easy to stuff some cash in the till and declare it as income with a car wash. Actually clean 100 cars a day? Buy enough cleaning supplies for 130 cars. Report on your records that you sold 130 washes. Dump the excess chemicals down the drain or just use them abundantly during washes. Sure, if you claim to wash a thousand cars when you only washed 100, then that could get you caught quickly. But if you're not greedy, you could keep such a setup going for a very long time.